hey i just started working out and one of my younger brothers has wanted to start with me and i'm worried that he is going to stunt his growth. but he's tall...so i don't know if that means his organs are already well developed or not. help would be appreciated. i don't want to damage my little brother. by the way...he's 15

Just start him off slow and fairly light to condition his muscles, joints, and ligaments to handle heavier weight later. Keep him in the 10-15 rep range for at least 8-12 weeks focusing on range of motion and form and he will still see gains. It shouldn't stunt his growth in any way, you just have to be very wary of injury for a body that isn't finished growing.

Thanks! And what about supplements? Should he take any or should he stay away from my creatine?

He should see gains come quickly in the beginning without supplements (extra protein like whey should be fine if he needs it). Hold off on the creatine until he's a little older and needs to bust a plateau.

Ok sounds good :)
Thanks.