Hi mnmeater,
There is no way for me to know this with any certainty as I mentioned before but the 28 would be the middle transition layer not the support layer (which is the polyfoam) and it would certainly be an improvement over what he is sleeping on now in terms of support based on “averages” at least and many people of his height/weight would prefer firmer yet (24 over 28 would still be on the soft side for his body type). As you can see in their chart here … 24 over 28 is what they call a “medium plush” and would be an “average” choice for many people who were lighter than he is and if anything I would tend to go a little firmer for higher weights either in the top layer or at least in the middle layer (the next level above 28 is 32 ILD).
I personally wouldn’t suggest considering a topper at all until you first have slept on a layering that is closer to what you need in the mattress itself and then if you need a topper at all (and getting it “right” with the mattress itself would be preferable over having to add another variable into the picture) I would choose a topper based on your actual experience on the mattress using the guidelines in post #2 here and the posts it links to.
The split layering is also a good idea for couples with different needs as Padinn suggested but as far as I know it’s only available in a king size (although you should always check with them to make sure as things can change over time) so if this is what you have it would be a good idea.
Dealing with only one variable at a time (the mattress instead of the mattress and a topper) is a much less risky approach.
Phoenix