I like your Momma bear approach. He knows he's messing with a bigger bully that way. Your simple and clear language left nothing unclear about how you felt. I also think the idea of giving him the job of protector has quite a bit of value. If you change his perception of himself, he can become the kid who protects others from bullying and make a big difference as he already has size on his side.
Dressage, you are quite wise!