There are quite a few on here that are anti-rooster, so at the first sign of aggression, they're ready to send them to freezer camp. I enjoy owning roosters.
You mentioned he's a cockerel, meaning he's not a year old yet. Depending on the breed, somewhere in the 9 - 14 month old range, most will go through a hormonal stage. This is when they start taking their role as protector very seriously. As with most teenagers, of any species, those extra hormones are not exactly beneficial to behavior. Bad behavior must be dealt with swiftly at this stage. Normally, it doesn't take very long. Keep in mind, you are trying to train him, not hurt, or injure him. It is at this stage, when I have one that's just begun acting up, I employ a rooster stick. A rooster stick can be a tree limb, an old broom/mop handle, or I use a 4 foot long 1x3 piece of wood. The rooster stick is mostly to give additional reach, since my arms are only so long.
Go into the run with your rooster stick, and walk straight up to your cockerel. Get him moving out of your way. Either tamp the rooster stick on the ground, or tap him on his butt, and get him moving. When he stops, pay attention. Is he giving you stink eye? Is he leaning on one shoulder? Walk up to him, and get him moving again. Usually in the beginning, it takes 3 - 4 times of getting him out of your way, before he comes to rest squarely on both legs, without giving stink eye. Now begin doing your coop chores. He may follow you, keeping a bit of distance, but he's not to sneak up behind you. If the females are all around you, and he joins them, that's fine, but he's to respect your personal space for now. In about 3 - 4 days, when he's showing proper respect, have your 9 year old go in with you. Show them how to enter, walk up to the cockerel, and get the cockerel moving out of the way. Have the 9 year old proceed with you doing regular coop things. The next day, go in with the 9 year old, but give the 9 year old the rooster stick, and have them proceed with getting the cockerel out of the way when they enter, and doing coop chores.
Normally, a couple weeks of this, and I find I don't really need the rooster stick anymore, BUT every time I enter the coop, I walk straight up to the cockerel/rooster, and get them moving out of my way, always, and I always make sure when they stop moving, there is no stink eye, or shoulder lean. By doing this, once they come out of the hormonal stage, they settle down nicely, and are a delight.