Sudden pitching slump

Needing some tips. Son is pitcher made the 9th grade upper level team. (They have 2 teams) He has always been a strong pitcher. Always made all star showcases. However on the other teams he was on he was starting pitcher. His strong suit has always been placement. Hardly ever walks. Pitching was where he excelled and loved to be, coaches joked he had ice in his veins never got rattled. He has now on a team with really strong good players. There are 2 kids that are really good and we get that. So he hasn’t been getting to start but was primarily the closing pitcher every other game. He finally got his shot last week to start. He started off hitting 2 kids and then the walks. I am not exaggerating when I say he in a season rarely walks anyone. It’s normally strikeouts or little groundouts, It didn’t even look like him up there, mechanics everything. He barely got started and was pulled. He has never been pulled as a pitcher before. He has been starting pitcher for last 3 years on his teams and pitching since kid pitch started.

Team coach and we’re good, said hey everyone has a bad night.
Went to go out and practice and still having trouble getting it back. Can a pitcher lose it that quickly ? He pitched the game before as closer and was great no problems. It happened almost overnight.

Obviously it’s a mental thing some background. He has been the closer and gotten to play second, but primarily he is only kid that is on bench for 4-5 innings every game. Every other kid he is alternating out if you sit more one game you play more the next game. Also we found out that the coach was starting pitch him but not letting him bat. Not complaining about playing time, this is highschool we get that. It was just strange because up until the other night any inning he has played in he performed no errors in field pitched great. 2 other pitchers have been pulled from games and they continued to play that game, my son was pulled and benched the whole game.

So question is best tips to mentally prepare him for next time he pitches again, he has 3 games next week.

I know he is not at level some of the people on board but any tips would be great.