I tie up up my horses but sometimes I don't have time to tie them up so I feed them from the greatest amount of grain to the lest amount (i have 8 horses) and they usually get done eating all about the same time. For your horses try feeding Cooper first then slowly walk to Aidens bucket and feed him. Then go over and see how much feed Cooper has left. Make sure he has at least half eaten and see if it works out feeding them that way. If not, I've heard of putting rocks in their buckets. If you have to try that with Aiden make sure you are using larger rocks (not huge but not small enough for him to eat)
Or you could try mixing hay with the grain that Aiden eats
He knows when you're happy, He knows when you're comfortable, He knows when you're confident, And he always knows when you have carrots.