Well, he's not oversized for an adult male boxer - but since he's just an 8 month old puppy, and not actually an adult... I think it's fair to say you're
probably going to have a very large boxer by the time he finishes growing. How tall is he?
There's nothing you can do about his genetics and the final size he's going to grow to - he will be the size his genetics dictate he'll be. But it would be wise to ensure that you keep him on the lean side

I know you've said he's not overweight, so not suggesting a crash diet here. But you're right that weight can be quite stressful on growing joints, so it would be a good idea to try to keep him as lean as possible (within healthy limits, of course). You should be able to see the last couple of sets of ribs on a boxer of correct weight, so keeping your guy at 2, maybe 3 sets visible would be a nice weight for him. His growth plates should harden up somewhere between 12-18 months, btw, once he's finished with all upward growth.