Hi rbrooks,
There are too many unknowns, variables, and personal preferences involved to use specs (either your son’s or a mattress) or “theory at a distance” to choose a mattress or a topper for someone else (see mattress firmness/comfort levels in post #2 here) without some specific reference points based on personal experience that can be used as a guideline but the good news is that if your son is already sleeping well on his Ultimate Dreams then that would be the perfect reference point and it would make a lot of sense to order a topper in the same thickness and firmness as the latex comfort layer in his mattress.
Phoenix