I have head some people say that for a baby ball (under 2 foot) that a 20 gallon terrarium is too large -they theorize that the expanse of space will freak the snake out and it will stress. Other people say that the snake will easily out-grow a 10 gallon terarium so you might as well put the baby in a larger tank right off the bat.
In my opinion, both opinions are correct. The ball python will stress in a vast environment if it has no place to hide. But you'll be providing a hide for him so that will be fine, and your snake ought to be just fine in there.
Now, if he gets larger than 3 1/2 foot long, I'd advise moving him into a larger terrarium than that, but that's just me.
Remember, ball pythons seem to really appreciate floor space better than height and they really appreciate good places to hide in.