If he is benching heavy weights, he should not take his elbows below his body. If he does push-ups the lighter weight will allow him to go farther for a little better range of motion. A lot of bench press is not great for baseball. Some is fine and benefitial. As for shoulder press, he should not have the weight behind his head as can cause shoulder impingement. If he does the shoulder press he should use an incline bench and keep the weight in front. His form is very important as are his pitching mechanics. A flaw in either one could lead to the types of problems you saw last year.