My take on reading through the original post was similar to CabbageDad's. I think "not getting in the lineup" means catching, but not hitting. Please correct us if we are wrong.
Has your son tried to talk to the summer ball coach about these issues? Not in a confrontational manner, but approaching it as a way to learn where his specific weaknesses are and where he needs to improve. If it were my son, I would encourage him (not me) to have that initial conversation with the summer ball coach.
My son has a high school coach that doesn't like him as a player much. He grew up around this coach and our families have always been friendly, so there is some history there. Sometimes, as a parent, it can seem that his treatment is a little unfair. However, it's a great lesson that life isn't always fair and you have to keep working hard and doing the things you can to improve. It can be a great motivator when looked at correctly. It's our job as parents to help our kids learn how to overcome these types of situations and continue to work toward what they want. He will earn way more respect from that coach by having the difficult conversation and making the improvement suggested than going above the coach's head.
Welcome to the site and good luck to you and your son!