Ways to tame an already 'somewhat' tamed gerbil?

It takes time and patience. Put your hand flat on the bottom of the cage for a while a couple of times a day (say, 5 minutes). He'll eventually investigate and sit on your hand. However, be careful not to move or pick up your hand at all. Eventually, he'll be comfortable enough to pick up. Never grab him to pick him up, and when you hold him, never enclose him in your hands. Let him run around your arms. Hold him like you're crossing your arms, but with both arms flat. In time, he'll get use to you. Also, be careful never to chase him inside or outside of his cage with your hands.