I can say my opinion about this as almost the same thing happened to me. Although, I wasn't going to be pitching on another team, but it was my first time on 60'6" field and I was actually a year and a half-ish younger (our baseball team was not very good). Anyway, it takes getting used to. At first it will seem really far, especially if he's not a very tall/big kid. I never really found my arm getting that much sorer... I didn't really throw the ball any harder, it just took getting used to the aim to throw strikes. Now the thing that sucked for me was that I was also our starting catcher, so in between throwing on the mound I would be behind the plate too, and throwing down to second base at that age was not good. My arm was pretty dead by the end of the season.