Work with your son as much as possible off of the field. That is where he will learn the most. You have to make it fun for him and work at his pace, not what you want him to be.
I griped and griped about my son throwing balls into my couch for 2-3 years, hitting the wall and putting dents in the sheetrock in the house, but, it helped him greatly with his accuracy and throwing motion. He is in 8th grade now and has a very strong arm. HE has to love it, and the more fun you can make it, the better he will get. Just don't be in a hurry with him and let his natural progression act out. I have spent thousands with trainers, but the best work he gets is when he wants to do it on his own.
My son started running cross country this last fall and is running indoor track, he has worked on his own and spent much more than time on his own than any coach could provide him. He went from running like a grandpaw to being top 10 in the state and top 20 in the nation this season in his age group. The athlete has to have the "want to" or it is all just noise.