What risks can the gym have for those under 18 years of age?

I don't think there is anything wrong with this - our son's HS has a weightlifting class. But I would make sure that he is set up for a few sessions with a personal trainer (usually from the gym) who can teach him about the equipment and show him the proper form and pacing, that is very important. A risk would obviously be injury to himself (or others) absent or without sufficient instruction. Otherwise I'd encourage him in this endeavor, also making sure he does not veer into steroid use, etc.

Hi - My kids are the same age and I had the same concerns. I hired a trainer at the gym for several sessions to show them how to use the weights correctly and provide feedback on what was appropriate for their age, size, growth, etc. Eased my mind. Happy lifting! 

Many gyms offer free summer membership for HS students. My current freshman son started going to the gym with his buddies last summer. Our local Planet Fitness is apparently a popular hangout for HS students in the summer for this very reason. In the Fall, we bought him a 24-hour Fitness membership where he can take in 2 guests. During the school year he takes at least one work out buddy a couple of times a week. I also signed him up for a free session with a personal trainer to show him how to use the equipment properly. I think this is a great way for boys to hang out and engage with their bodies. The weight lifting is also recommended by his tennis coach to build strength. 

Agreed, nothing wrong with lifting weights at 17 as long as he is supervised. (Same goes for adults new to weight lifting, for that matter.) If his school doesn't offer a trainer who can support on this, try a local gym (I think even the Y has classes) where he can sign up to work with someone--ideally someone who has worked with other kids his age.