Baby buns can leave their mums at 8 weeks, male bunnies become fertile at around 12 weeks so should be separated sometime between the two. Ideally though, bunnies should never be kept alone so it may take a bit of logistics to work out who is safe to go with who!
Female bunnies don't usually get fertile for another couple of months (but don't bank on it!) but a boy could definitely get his mum pregnant from around 12 weeks! Hope that helps.