He is adult, so you cannot make him force to take his meds. Maybe there are other reasons that the sexual side effects why he doesn't want to continue with his regimen and if there is... you should acknowledge that....

Stoping cold turkey is dumb thing to do, because withdrawal is a nasty thing... But again... it is his decision. YOu can let him know how is actions affect you, but you cannot make him do anything.

Just have a talk, like two adults. Don't be too flully or condescending.