I agree with Chrissy - introduce them straight away.

My breeder gave me a good tip...............
When you go to pick your new kitten up, take a blanket with you that your other cat uses. Then when you are about to take new kitty home wrap her in the blanket so that she no longer smells of the place she came from. She will now smell of your house.
That way she wont smell as strange to your boy.