Hi lightsidesleeper.
Welcome to our mattress forum :)!
Keeping in mind your son’s chemical sensitiveness, BMI, and sleeping positions, the mattresses you are considering are good value/quality options that could work given that it matches your son’s comfort preferences and alignment needs. I am not familiar with the old Ikea bunk bed you mentioned but I’d assume from your general description that it has rather large unsupported areas that would be too large for an all latex mattress or some other types of mattresses which can easily cut into an all latex mattress (what many manufacturers will refer to the grids as cheese slicers). There are some creative solutions that people have found such as adding a coir bed rug or vinyl lattice that you can read about in this mattress foundation thread here.
All manufacturers are different in what they recommend for their products. The first thing to do is always to check with the mattress manufacturer you’re considering and ask their opinion to make sure that the bunk bed is appropriate and doesn’t void your warranty. My Green Mattress’s description of Kiwi states that “it works well on any traditional foundation, including solid or slatted platform beds that are no more 3.5 inches apart” but of course the contact grid/mattress area is important in distributing the load of the mattress and MGM would be able to assess if the foundation is suitable for the mattress.
It would be helpful if you’d have the name of the Ikea bunk bed you own, to see if someone who owns one knows the weight capacity specs for your model and can support the weight of the mattress and of the sleeper on it.
You certainly are considering a few good/quality options and I’m looking forward to hearing about your final decision.
Phoenix