I said it in your other thread and I'll say it again, 50 gallons is way too small for a common pleco, especially a 12" one. The reason why yours doesn't hide isn't necessarily because he doesn't want to, it's because the tank is so small that it's impossible for him to. Yes, 50 gallons is still bigger than the 30 gallon he was in before, so naturally he's happier, but by forcing him to stay in a tank too small you are causing his guts to all grow into each other.
Do some research on what happens when a fish is kept in too small a tank. It's painful, and it causes serious health complications as they get older. If you truly love your pleco, do what's right for him. It hurts, I know, but you're shortening his life unless you're willing to upgrade to a 120+ gallon tank. If you do re-home him and want another pleco, look into bristlenose plecos or one of the fancy, smaller species. They'll be perfectly happy and healthy in your 50 gallon.