I have seen cockerels as young as 10 weeks old start to have an attitude, so I would say this is normal and sometimes unfortunately just happens. If they weren't raised together from hatch then that can also make it more difficult. If at all possible I would say(depending on your setup) keep them separated for awhile. In a way that they are still in their coop (or wherever you are currently keeping them) but are separated by poultry netting or chicken wire. That way they can still see each other, but they can't get to each other. Each day let them out on neutral ground(the yard, etc) where they can have the opportunity to spend some time together. When "play time" is over, put them back in their separate sides. Eventually the goal is to get them used to seeing each other without having the aggression issues. Eventually you can just remove the divider once they are getting along again. Some cockerels just get worse from this point and there's nothing you can do most times. Make sure to have plenty of hands on time with them both, especially the little roo. Are these the only two chickens you have? I've also found that younger cockerels seem to lose some of their confidence when there are a few more girls. They tend to keep him in line, lol. What breed of rooster and pullet do you have?