Short of a court order, you can’t force treatment - unfortunately.
And, if the treatment is forced, the person usually stops it as soon as they can.
Everyone recommends a book called I’m not sick, I don’t need help by Dr Xaviar Amador.
It teaches the LEAP method to help persuade the person to accept treatment even if they never acknowledge they are sick.
I ordered my copy off Amazon, but there are online videos as well.
This link has some good information about it - it’s another thread on this site:
If he becomes a danger to himself or others, you can ask for him to be evaluated for involuntary hospitalization - that’s usually up to a 30-day hold, but people rarely stay in the hospital for 30 days.
The best thing to do is learn what the laws are in your state about involuntary hospitalization, etc. so you’re prepared. NAMI is a good resource. So is your local crisis hotline if you have one.