Give him time. He will have to earn their respect, as well as learn to be a rooster. Your birds are all young yet, and I don't consider anyone mature until they are 2 years old.
Silkies mate like every other rooster. He may have to get creative as he's probably smaller, but I keep bantam cochin roosters with standard breeds and they figure it out.
He could be attempting to mate or dominate them. He's still a stranger. Expect it to take a couple of months before they become more cohesive of a flock. Eventually the hens should like that he's there.