I wasn't sure what the best age would be
Anytime up to 9 or 10 months is fine (younger is probably better, as they are less wiggly). You don't want to wait longer than that, or their feet will be too big for the container.
I'm afraid of doing it and him moving and messing it up.
You really can't mess it up by him moving. The mold will set in whatever position his hand or foot is in. The tricky part comes when you fill it with the plaster mix...you have to make sure you fill all the little toes and fingers.