He's already done his first cycle, and it doesn't sound like he's open to suggestions. Not sure what you expect anyone here to be able to do for you. There are a zillion threads on here where I and others have advised against what he's doing. I guess you can show him those. But if you couldn't convince him not to do the first cycle, I don't see how you're going to convince him to stop now. Sorry man. Best advice I can give is to take care of yourself. Don't be stupid just because your friend is. He MIGHT get away with running gear the way it sounds like he plans to do it, but in all likelihood he'll be blasting and cruising well before he's 25.
I'll say 1 more thing that might help. 17 is too young. Without a doubt. I would never, ever suggest that a 17 year old should run gear. Ever. Ever ever ever. But even more importantly, running gear with essentially no training experience is an AWFUL idea. EVERYBODY should learn to train, for YEARS, before considering anabolic assistance. 3 months is insane. Your friend will never understand diet, he will never understand proper training, and he'll just keep raising the dose instead. We've all seen it before. I'm as strong as I am now because I learned these things FIRST. There's no way I would have gotten those things right if I had jumped on steroids early. You can tell your friend that he should wait at least a year or 2 to run his next cycle. Learn the things I'm talking about here. Experience training naturally and learn to maximize gains through manipulation of training and diet. Those 2 factors will ALWAYS be more important than drugs.