You need to tell him its his decision and whatever he decides to do you will support.
With my youngest, I was totally bumbed out when he decided to stop playing all his sports but after talking with a few people I let him make the decision. He wanted to do other things and really learn computer program and design. He was so happy that I took off the pressure and let him do what he wanted to do ..
His love is more towards computers and music and I have supported this all the way so instead of him graduating next season headed to a college to play a sport, he will graduate HS with a diploma, a Cisco certification, a A++ certification and will be close to taking the Microsoft certification test and will probably make more money at 19 than many of us made in our thirties but even more importantly he is happy about his decision and has never looked back.
As a parent you have to realize when its time to let them make a decision based on when they hit a certain maturity level. You want them to be happy and you want them to know you trust them and respect them enough to let them decide.
I know you and I know your older son.. there is no doubt in my mind you will do what is best for your youngest.
(MWR - yes I am referring to your old 10 yr old's catcher.. can't beleive he and yours are just a year away from graduation)
_______________
"Baseball, it is said, is only a game. And the Grand Canyon is only a hole."
"JustMom"