Here's another pitcher you and your son might want to have a look at...only the most feared RHP of his day:
Are his mechanics solid enough to reduce (i know there is no precluding) injury?
There is no "special" threat of arm injury to a sidearm pitcher. In fact, kids who try to please coaches that tell them to "get the elbow up" and "throw over the top" may be at greater risk than kids who simply throw with whatever release point comes naturally and feels right to them. The reason being: An accomodating kid who tries to blindly follow the "get the elbow up" advice may very well go too far--there is good scientific research to show that a pitching delivery in which the elbow actually gets higher than the plane of the shoulders (the acromial line) can be a big problem.
If he is a sidearm, where is a good resource for training exercises? I did a small search on this site already.
There is no special training/conditioning for sidearm pitchers that I am aware of. Depending on his true level of interest in pitching, start getting him involved with consistent strength training that is appropriate to his age/development level. For kids at 11 - 13 yo, this would usually mean isometric work like prone holds, that strengthen the shoulder and subscapular muscle groups, regular push-ups and "triceps pushups" to tolerance, cariocas without weight....that sort of thing. Most coaches don't recommend weight training until after puberty...after 13-14 yo, there is plenty of good stuff for dedicated conditioning and training of pitchers in Tom House's books.
- Any advice?"
Gonna sound trite, but have fun with him and let him keep it fun right now. Any conditioning work he does at this age will be inherently boring, compared to pitching, so you can help with that by doing the work right alongside of him. My kid used to laugh uncontrollably at my feeble attempts to stay in a prone hold for longer than about 1 minute...anyone who has done them will know why.
If you are able to swing it financially, look into spending a week of your summer vacation in Los Angeles at the USC campus where both you and your son can have your lives changed at one of Tom House's pitching clinics. He's the best in the business, IMO, and it's always a wonder to me that pee-wees like your son can get the same quality of hands-on training from Tom that his numerous pro clients get.