Ariana, he should know they babies aren't big enough to breed. When they are ready they will squat for him and their combs will be developed. I have 23, 2 month old pullets running around and my roo just watches them, never is aggressive. You will probably have to stay around and watch when they are together. Do you have enough extra fencing materials/chicken wire to divide up the pen so they can see each other from their own area, but not get too close? Also, I agree with the other posts, you need to cover the run. A hawk will carry off a young pullet given the opportunity.
Hen saddles or aprons will prevent the roo from breaking all the hen's feathers on her back when he mates. Typically a roo's favorite hen/s will have broken feathers on their backs and if bad enough bare skin showing which can lead to open wounds if he continues to mate her. You can find patterns or directions to sew them online, probably even on BYC, if you sew or have a relative who can help you. They don't take long to make and of course you can buy them.
Good Luck!
Diana