My son had the same injury, bilateral pars grade 1 with 3 mm of movement. We had three spine specialists say it would always be a problem, good luck. Ran into a chiropractor who said he could help. Put him in a brace and prescribed PT. That was almost 9 years ago. The kid plays DI baseball. We talked to a grandparent at a tournament who said he had the same and didn't know until he was 42 and had some issues with numbness in his leg. Andre Agassi won four grand slams with a grade 2.
The chiropractor said if his back hurts due to pitching either he has bad form or he's not meant to be a pitcher.