Is it ever acceptable to say enough is enough

I've posted before about my youngest DS and his numerous problems with being bulimic, he's 43 and has had this horrendous eating disorder since being a teenager, he was always a funny friendly lad, he's now a man full of rage and moody beyond belief, I'm 100% sure this broke up his first marriage, he treated his first wife dreadfully, I'm ashamed to say he was everything I hate in a man..... he remarried and has two darling baby daughters, but I see him acting the same way as before, today once again our DiL came round terribly upset, he's like Jekyll and Hyde,and needs professional help we've talked and talked I've been to doctors with him and counselling but he doesn't keep it up, he has Prozac medication but stops taking it cause he says he's fine, I read lots of advice on here where you very wise folk, say be there for your family member and support him when needed......I must be a mean cow cause I'm sick to death of him, if he won't help himself how can we help him, .....I'll do anything to protect our DGC but surely it's ok to say enough is enough where he is concerned