Try rewarding him with a little piece of cheese after he's gone and make a really big deal out of it - I mean - when he's gone outside.
You said "when I think he's gone?" Are you watching him and making certain that he has gone before you bring him in? Keep up with the bell, rewarding him, and make it a really big deal and get super excited when he has done well and make over him like he's the best lil guy in the world. He will get it.
Are you helping him ring the bells on the door each and every time you take him out? They usually get that really fast.
He's young yet, so don't give up. Just be consistent.