Probably about 8 weeks, although it's hard to tell for sure from a picture. Kittens really should stay with their mothers for 10-12 weeks, but if he doesn't know how old the baby is I assume he doesn't own the mama cat.
:lol3: Willowy, I've usually pretty good with guessing sexes just from their appearance! Only once have I been wrong... I'm thinking the OP has a boy. But honestly there is absolutely no way we can tell from this side.