First, I have to laugh because that is kind of a funny problem to have.
Second, I'd say to work on an actual Stay command so that you divide up the behaviors 1)sit 2)stay 3)release.
He should catch on to stay very quickly since he is already doing it. You just need to label it and practice it with lots of different settings and distractions. That way you can make the release actually rewarding.
Also, to get him to release his sit he has going now, toss the treat away from him along with saying "OK!" good dog so that he has to get up to get the treat.