I'd hire a dog trainer to tell me what's going on specifically. Maybe your younger son is more spastic in his movements which cause your dog to be unsettled around him. Maybe he smells a certain way which triggers your dog.

Use a can filled with pennies when he barks. Shake it.

Barking isn't necessarily aggressive. Sometimes it is a sign of excitement. Sometimes it is about boredom. It is communication. I would not operate from the premise that the dog dislikes your son.

I really don't know what to do. I have a 10 year old and an 8 year old. Our dog will just look at my younger son, especially in the morning before school and just bark at him - never my older child. If my younger one has a friend over, the dog barks the entire time at the friend. I am guessing it is some kind of power play- but don't know what to do about it. Any suggestions are appreciated!