You wouldn't be losing him sliver...
Believe you me, you can bet he'll be wanting to come back before long.
I had a similar situation, but reversed, with two stepsons. They rebelled against their mother around the same age and wanted to come & live with their dad and me. It worked out very well - one went back to his mum eventually, the other never did - but both have a good relationship with all of us now they're grown up.
You'll miss him, he'll miss you - but I think you have to give it a chance. My bet is, he will be back within a year!