Hi tmstuber,
The first place to start your research is the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that can help you make the best possible choices … and know how and why to avoid the worst ones.
Post #2 here also links to the better forum posts and topics about mattresses and children that include more information and guidelines about buying a mattress for a child.
The better options and possibilities I’m aware of in and around the Champaign area (subject to the guidelines here) are listed in post #2 here.
You can see some comments about the Green Choice mattresses in posts #9 and #13 here. They use lower quality/density memory foam and I would tend to avoid them because of potential durability issues and because I would tend to avoid memory foam for children.
I would also tend to avoid chain stores and the major brands they carry (see the guidelines here).
This is certainly a low price but the quality/durability of the foam will depend on its density (see the foam quality guidelines here). For a lighter child I would tend to look for a minimum of 1.5 lb polyfoam and higher density would be more durable. It could make a reasonable choice if the foam is a suitable quality/density and firmness.
Again I would tend to avoid memory foam for children and at this price you can be assured that it would have very low quality materials.
I’m guessing that the mattress has an innerspring but not a “box spring” built into it and it will still need a good foundation underneath it. Again at this price the mattress you can be sure that it uses very low quality materials but I would need to know the specifics of the materials and components inside it (see this page) to make any more specific or meaningful comments about it.
I would be aware that the warranty on a mattress will have very little to do with the quality/durability or useful life of a mattress or how quickly you will need to buy a new one (see post #174 here). The most reliable way to assess the quality and durability of a mattress is by knowing the specifics of the materials inside it. The Green Choice mattress is very unlikely to have a useful life of 15 years or anything close to it (under 5 years years would probably be closer).
Again I would tend to avoid the Green Choice mattresses and most other memory foam mattresses in such a low budget range (unless you can confirm that they use higher quality materials) and I would tend to avoid memory foam for children anyway. You will likely need a good foundation or bunkie board to go under the mattress for any mattress you purchase.
This could be a good choice depending on the specific mattress you mean but I would keep in mind that the mattress doesn’t have a “box spring” built into it (I don’t know of any of the Denver Mattress models that do) and you will need a suitable foundation under the mattress. Children do better with firmer mattresses so if you do decide to add a topper I would choose a material other than memory foam until they are older and mostly finished growing. A mattress protector (but not necessarily a mattress encasement) would be a good idea for any mattress you purchase. There is more about the pros and cons of the different types of mattress protectors in post #89 here but I would lean towards one of the thinner membrane type protectors in a lower budget range.
What your son wants and what will best serve what he needs may be two different things (children love to make choices in many areas that aren’t the best choice for them). There is also more about the different ways to choose a suitable mattress (both online and locally) that can help you identify and minimize the risks involved in each of them.
Post #13 here also has more about the most important parts of the “value” of a mattress purchase that can help you make more meaningful comparisons between mattresses.
You are in a very low budget range where most mattresses will use some very low quality materials and you will need to make sure that the quality of the materials in a mattress are as durable as possible.
The foundation post here has a few DIY designs for platform beds that may be of interest (near the end of the post in the platform bed section).
Phoenix