Your rooster is actually a cockerel and is full of raging hormones that are telling him to "breed, breed, breed"! Your older hens aren't accepting him because he's just a punk and they're waiting for him to earn their respect (which he may do as he matures). If you have pullets around the same age, they're likely rejecting him because they're not as sexually mature as he is. Since he's young, if you want to keep him, you could put him in a separate pen from the hens/pullets until he matures more. How many chickens do you have, and how much space do you have them in? It may help if they can get out and free range, or if they have more coop/run space to avoid him.