Wow, that's a pretty bad scar on his head. It's healed over so there really isn't anything you could or should do for him. It's nice to see that you've been able to give him a good home, though. He looks healthy and has nice colors/blushing.

Oh, and the scar looks like it could have been caused by a bad rat bite or he could have scraped himself on something in his cage. You just want to make sure any sheds around the scar come completely off.