The money issue IS a major concern as all of both your and his money will pass to this (or another) other women if/when husband remarries. Without very specific provisions (realistically, a far in advance pre death gift from your husband to your son), you son would inherit zero from your husband.
Zero.
This is a very, very common scenario. You simply cannot bank on your husband doing the right thing because of his love for his son. In assuming so, you would he protecting your own care for your son onto your husband - but look at who and what you're actually dealing with. A man who cheats and traumatises his family whilst his son is gravely mentally ill and his wife dying.
I wouldnt be surprised if your son's illness stabilised with distance from such a man. Indeed, I would expect it.
You need to ensure that your more than 50% of the marital pot (probably 70% on needs basis) goes directly to your son. Due to your son's age and current ill mental health, I would do this in such a way as to stagger the money with essential life costs from 18 to 30 and balance of estate at 30 years old (a trust structure). This structure is more realistic if you are divorced and have full control of your own estate.
Make this your final and most loving gift to your child - the gift of setting him up to be financially secure for the rest of his life. With each annual payment and each time he walks into his own home (with equity from your estate), he'll be reminded of and wrapped up warmly in the love of his Mum.
💐