What should I say to him?

So my boyfriend is taking a lot of supplements for working out and trying to get results very quick. I'm nervous about all his taking, I want to spend the rest of my life with him and do not want him to be causing himself harm and not realize it. Plus, the testosterone makes him act differently and it's not a good different. I've always told him how I felt about it, but would never in the world tell him he has to stop. But we talked about it the other day, and he gave me options. One included him stopping completely, the other just taking one and another just taking another. I, of course, want him to stop completely and I think it would be good for him. But I'm afraid I'd be making him do something he didn't want to do, or he'd end up resenting me for it. What should I do? I don't want to make him do something, but I also don't want him to end up hurting himself.