P.N.
He's really too young to learn to be gentle with the kitten. Even if you put him in time out constantly and he does get the idea that being rough is wrong (and he still might not), that still does not mean that he will be able to control his impulses to be rough.
I think you need to keep them separate unless you are right there with him every second. We have 3 cats, one of whom would not flee the baby (the other 2 always did). Even after being scratched by the cat a couple times (when I was not on top of keeping them separate), she still would reach out to the cat. So even actual wounds did not give DD the ability to control her impulses with the kitty.
He's just too young at this point and he could really harm the kitten.