Hi lbahy,
Welcome to the Mattress Forum! 
Besides the characteristics that are important to you, sleep ergonomic research will generally recommend something that is in the “medium to medium-firm” surface comfort range for a growing child, due to the postural and epiphyseal plate formation that goes on during this time frame. You may have read this already but just in case you haven’t … there is more information in post #2 here and the topics it links to about mattresses and children and “suitable” and “safe” materials including a link to some general guidelines for children in post #2 here. It also includes a number of links to the better forum posts and topics about mattress and children as well which have more information about many good options children which would be well worth considering.
[quote]I am looking into an all natural latex mattress with the following:
Hypo allergenic[/quote]
There are no Federal standards or definitions that govern the use of the term “hypoallergenic.” The term means whatever a particular company wants it to mean. Manufacturers of products labeled as hypoallergenic are not required to submit substantiation of their hypoallergenicity claims to FDA.
With that being said, most people that are looking for a “hypoallergenic” or “natural” mattress or materials are usually concerned more with “safety” than whether the materials have an actual organic certification and they usually aren’t aware that an organic certification isn’t the same thing as a safety certification. There is more information about the three different levels of organic certifications in post #2 here and some of the benefits of an organic certification in post #3 here and there is more about the different types of organic and safety certifications such as Oeko-tex, Eco-Institut, Greenguard Gold, C2C, and CertiPUR-US in post #2 here and more about some of the differences between organic and safety certifications in post #2 here and there are also some comments in post #42 here that can help you decide whether an organic certification is important to you for environmental, social, or personal reasons or whether a “safety” certification is enough.
If you’re interested in more information about the different types of latex sensitivities and latex allergies, this thread is quite informative. I am supposing that you would already be aware if your son had a Type I latex allergy, which is very rare.
Dust mites need both food (primarily dead skin cells but other foods as well) and a fairly high humidity/temperature and since latex of any kind ventilates better and releases moisture to the air better than other foams this alone would make any latex less hospitable to dust mites than other less breathable foams. Dust mites also seem to prefer food that has been partially decomposed by fungi so the documented anti-fungal properties of natural latex may play a role here as well.
I have also read many websites that say natural latex itself has some property that makes it inherently resistant to dust mites and it would make sense to me that if this is the case then the higher the natural latex content the bigger any effect would be but I haven’t seen any specific evidence or studies to back this up and I believe that it’s anti dust mite properties are more the result of its breathability and ability to control humidity.
A pillow is also a bigger source of problems for dust mites than a mattress and is the first place I would start in any efforts to reduce dust mite populations and reduce or control the allergies they can trigger. Regularly vacuuming the mattress cover can also be very helpful along with regularly washing your sheets and bedding (and a mattress protector) in hot water which can kill most dust mites and rinsing thoroughly (twice) can significantly reduce the presence of dust mite allergens in the sheets and bedding ( see here ).
Latex can again be a good choice for this. In very general terms … the materials, layers, and components of a sleeping system that are closer to your skin will have a bigger effect on airflow, moisture transport, and temperature regulation than materials, layers, and components that are further away from your skin and softer mattresses or foam toppers will tend to be more “insulating” and for some people can sleep warmer than firmer versions of the same material. There is more about the many variables that can affect the sleeping temperature of a mattress or sleeping system in post #2 here that can help you choose the types of materials and components that are most likely to keep you in a comfortable temperature range.
Most component mattress systems are not meant to have the cover removed and washed frequently, especially ones where the cotton and wool are untreated and quilted together. This is instead the job of the mattress pad, protector or encasement. I would consider a mattress encasement designed for allergies as allergies are already an issue. There is more about mattress encasements and allergies in post #2 here, and information about mattress protectors in post #89 here and this post. There is also a good study here which clearly indicates that an allergy cover will reduce the dust mite population but by itself this won’t be enough to have a significant effect on allergy symptoms.
[quote]Customizable components
Natural top cover (wool?)
Dunlop bottom
Talalay top
Decent warranty
Flexible return policy[/quote]
There are many choices available for latex mattress component systems, including quite a few who are members here of the site. You may wish to look online and quite the expertise of the members listed in post #21 here who are all very experienced and knowledgeable and specialize in providing the type of help and guidance on the phone that can help you make good choices. There are a wide range of latex component options included in the choices there and I believe that all of them compete well with the best in the industry in terms of their quality, value, service, and transparency.
This would be personal preference, but you are correct that it is common for people to choose Dunlop bottom layers and Talalay on top, as Dunlop tends to have a bit more of a “dense” feel to it and Talalay tends to feel a bit more “buoyant”. I would advise you to phone any manufacturer you are considering and use the guidance they provide for you particular situation, as they are quite experienced with requests like yours and they can also provide for you the pros/cons of the different layering systems they offer to help you make an informed decision.
I think you already have a very good start in that you are considering creating a mattress that uses high quality componentry and one that can change over time to meet your son’s needs. I would take a bit of time to read through the links I’ve provided, including the mattress shopping tutorial here, and then spend some time on the phone calling a few of the manufacturers that you are considering and use their advice, along with your own preferences, to make a final decision.
I’ll be interested in learning what you decide to do.
Phoenix