You can definately move him now.
It's not that a 10 gallon will hurt him, it's more that they're so small they'll barely last a month before needing an upgrade, so it's generally reccomended to just go ahead and start with a 20 or 40 gallon. I usually reccomend starting with a 40. You can always section it off if you need to, but since you've got the 20 gallon and it's big enough to need sectionning, you can definately move him now.