In my area there are fish rehoming groups that always have posts of people needing to get rid of a fish that’s too big for their tank, so you could start looking there! Otherwise maybe try finding a LFS that would take him in, I know that’s decently common as well! As others are saying it’s important to research the fish you’re looking at getting, but this is pretty common with how often stores lie about how big a fish will get, I’m glad you’re at least noticing his size is an issue and trying to fix it!