I'd try to see the two things as being separate challenges.
One way to approach them is to do a grade skip if it's offered and do club sports, which are usually based on age. So, for example, my DD has a grade skip and does a competitive sport with a local gym with kids her age.
That said, your ability to do something like this depends on what kinds of sports your son likes. Sports that are often offered outside of schools include swimming, gymnastics, and soccer. If your son wants to play football, basketball, or baseball (which sounds like the case), it might be more difficult.
Has a grade skip been offered? If so, what's his opinion?
