If money is no object then I would get your son citizenship. You never know what the future holds and he may well want to work/live over here in the future. This will be his only opportunity to get citizenship unless he comes over on a works/student visa or marries a UKC.
Personally, I would jump on the opportunity before his ILR expires.
The problem with this is that because he is over 18, he has to make his own application for citizenship and meet all the regular requirements... and because he has been living outside the UK, he is not going to be able to qualify this year.
If he was under 18, he could just apply for registration as British under form MN1, based on his mother applying at the same time... he would be entitled to citizenship purely because his parent is applying for it.
However, once he is over 18, he has to qualify in his own right and apply separately on Form AN. And in order to qualify he must show:
- he has held ILR for at least 12 months
- he was physically present in the UK exactly 5 years before his citizenship application date
- he has not been absent from the UK for more than 450 days in the last 5 years
- he has not been absent from the UK for more than 90 of those 450 days in the last 12 months before applying
If he has been back in the US for almost 2 years now and has not returned to the UK at all, he will definitely exceed the 450 days AND he will definitely exceed the 90 days in the last 12 months - there is some leeway on both of these, but the leeway only extends up to 180 days in the final 12 months, and he will likely have been gone for more like 300+ days.
So, I believe, if the OP's son wants to be able to qualify for UK citizenship he will need to:
- move back to the UK permanently before his ILR becomes invalid this summer
- live in the UK for up to 5 more years... however long it takes so he meets the residency requirement for citizenship