Hi DebKC,
As you know I think highly of Arizona Premium and the mattress you are looking at would certainly make a good quality/value choice.
There are some guidelines for children’s mattresses in post #2 here but both of the options you are considering (28 ILD - 32 ILD) would be in the range that would be suitable for a child. I would keep in mind that 28 ILD is only “soft” relative to support cores and wouldn’t be considered to be a “soft” comfort layer.
I don’t think that you would go wrong with either of them and if you are looking for a mattress that will last until he goes to college I would make the choice based on how large you expect him to grow as he develops. If he is likely to grow to be average to heavier than average then the firmer (32 ILD) choice may be better because it would be more “supportive” for an average to heavier adult and you could always add a softer topper if it becomes necessary when he grows older and his body develops into a more adult shape and weight distribution if he finds he needs or prefers a softer sleeping surface with less risk to alignment than a softer base mattress. The softer choice (28 ILD) would also be fine if he grows to an average to lighter body weight and may not need a topper at all when he gets older since it’s already a little softer.
It would be difficult to go wrong with either one IMO since both of them are in the medium to medium firm range.
Phoenix