His growth plates shut. Probably won't get any taller.
His endocrine system is not yet matured. This could kill his sex drive. Give him a long lasting limp dick.
This could **** with his prostate. Mostly effecting sexual pleasure.
This could permanently negatively effect igf1. This means once he comes off...he would have a hard time building muscle because igf1 is one of the primary paths.
If he has underlying conditions like diabetes, heart disease, high blood pressure, hormone related illness or lipid problems this could speed up the progress.
He will most likely have colesterol imbalances which could lead to gallstones which could lead to death or surgery.
He could aggravate a mental disorder. Especially considering when you take anabolic steroids certain neurotransmitters are slightly effected.
He could get liver damage (usually temporary but can be permanent given underlying conditions)
If your endocrine system is not matured at the time you take that step to hop on you're more likely to have permanent low t.
One thing is for certain. At 16 he has no idea what he is doing. He's probably going to gain a few pounds....get off and be the exact impatient person who made the decision.
I would suggest anyone who makes that decision to carefully research. Be at least 21 years old, 4 + training(consistent) under your belt, a vast knowledge of Nutrition and the willingness to stay on year round. You do gain new nuculi that will increase your natural genetic limit to a great extent. However.... Once your off the gains in size you made go away.although you keep the nuculi Even with pct. Due to decreased igf1 and your androgens re balancing. It takes anywhere from 4 to 8 months for this to happen. Staying on year round is pretty much the only way to get the gains pro bodybuilders get. It does take years.
For instance .The first 4 to 12 weeks on testosterone your weight Wil skyrocket. After that your body has adjusted and gains slow. This does not mean it's not working. It just means your androgen receptors have....caught up...so to speak. 2 lbs of lean mass a week is still possible potentially. However....The 3 to 8 lbs a week you got giddy for will be gone. Which why most people think "I'll just switch compounds"
It's a life long choice. It's costly and hard.
Not to mention If you don't take peptides or gh along with it you will eventually cost yourself your joints which leads to chronic pain and suffering.
BUT YOU'LL GET HUGGGEEEE
He's made the choice. It doesn't mean he's a bad person. he's still your friend and he puts the same work in as you in the gym. Just be there for him when he either crashes and burns or he turns out okay. It's his choice. He knows the dangers if he reads this post.
I'm 23 and I'll come to a point where I consider it. Just not for many years.