you can't force someone into treatment, but you can take care of yourself so that his problems don't become your problem by gonig to therapy for yourself that way at least you have someone to take to and help you though this and also if worse case scenio happens and he becomes suicidal and so on you have at least one professional that can help you take the nessesary steps to get him involuntary help, he will be very angry that you and your therapist took those steps but in some cases better angry then dead.

Hang in there and welcome to psychcentral.