Hmm, when changing names just consider to keep it simple and easy, and let them get use to it. But you can also go through various names to see which they respond to best
Usually something that's 2 syllables and ends with a "y" or y-like sound. Like Toby, etc. But if anything whatever you prefer, you just may have to train them to recognize it. Otherwise, if his previous name seems to work best then it won't hurt to go with it ^^
There's plenty of suggestions in past posts/thread but here's some of my suggestions. Toby, Max, Mick/Mic, Zax, Zack, Jackie, Joe, Joey, Mickey, Mike, Mikey, etc.