My son decided to take a placement in a smaller collegiate summer league mainly to see college pitching and to get experience managing life on his own. Helped that his placement was in a really fun little city in a great State (Boulder, CO), far enough away from us to garner him some real independence. I thought it was a great decision and totally supported him.
I think a lot of your decision should revolve around the extent to which his present summer is competitive etc. I just don't think in our case that spending yet one more summer racking up ABs against kids going into HS Jr or Sr years was going to help him get better.