something like the .38g will probably be ok for now, they're pretty cheap too.
When he gets bigger you may want to consider moving him into the next size up.
I think a 3 month old is ready for screen cage.
I put my 3 month old veiled and 2.5 mo panther in a screen cage already (they are cheaper and easier to clean).
as far as size, i use 24X12X24 (inches) cages.
