Honestly, if you have had conversations and teaching/coaching moments with him to get "buy-in" to your philosphy, and things are not changing, the best thing for all involved is for you to let him go. I know this can be difficult, but in the end everyone is happier and better off.
However, having said that, you might try explaining to him that you feel the program & you need his "expertise" with the 10&unders. You think he can really help the 10&unders.
How is his communication, relationships and effectiveness with working with parents? If pretty good, you could tell him you also need him with this very important group of younger swimmers & newer parents to help educate & guide them...his experience can help them.