I think your husband’s weight is only the tip of the fatberg.
He’s basically refusing to take care of himself. (Think what little patience MNers have for eg a cocklodger who refuses to look for a job or pay his way.)
Would it be possible to frogmarch him to the GP for a health check?
I think it’s perfectly acceptable to tell him you won’t even consider sex with him until his BMI is under 25.
You might have to clear all the junk food out of the house and have everyone eating healthier.
If there IS an underlying cause for the overeating/unfitness, it’s absolutely up to him to take steps to sort it and talk to you about it.
I’m sorry things are so hard for your son.
However, you have to make plans and take action if your husband is unwilling to change.
An ultimatum without consequences is, after all, nagging.
Hugs though, it sounds rubbish.