Just some simple mental approach suggestions...
-Celebrate in the success of his teammates ("..he's pushing too hard to hit HR's because last year he's the one who was hitting them... this year, other kids are hitting them"). You don't need the extra pressure on yourself. You do need the hitters around you to continue to succeed in order to start getting pitched to again. That's probably why it is starting to happen.
-Each pitch, take the approach of "I'm hitting 'til I'm not". Have a clear focus on the zone you will fire on and expect the pitch to be in that zone.
-Forget about the HR's, just hit the ball hard where it's pitched.
It's quite common for a returning successful HS hitter to be pitched differently and/or around. It's a game of adjustments. Take what they give you. If that is walks, take them. Eventually, there will be no room to work around you - be ready for that. If they found other weaknesses (wanting to pull the ball out of the park), they'll try to take advantage and pitch away. Adjust and drive the ball the other way.