You have to explain the expectation clearly, otherwise the kid may get really frustrated. One of my son's friend, a 6'3 200lb righty as a 15 yo, got to play with our 17U team last summer. Exactly like yours, he's been told that he'll be PO and will not field or bat if he plays with the 17U. He liked the idea of playing with older kids and the honor/bragging rights it brought. However, at the end of the season, I met his parents and they told me that he's not happy and will quite the team. He didn't expect that as a PO, he only get to play once a week, and will be sitting on the bench all other games. He asked the coach many times to bat, the coach just stared at him and said "no". Once he went to batting warm up with other players, and coach called him back and said "what are you doing here? let the batters warm up." One time he was so bored sitting in the dugout, he was chatting and laughing with another pitcher, the coach called him out and said "shut up and pay attention to the game!"
Anyway, he quit the team and joined another (probably lower tier) team when they promised him that he'll get some at-bat. I think he's just not ready to be a PO, although his body type and skill level says he should be a PO eventually.