Our son, in middle school, had a blue belt in tae kwon do. A bully twice his size followed him around all day kicking at his feet and talking trash at him. Finally our son decked him. Down he went. Our son walked away. I still think fondly of that moment.
You made a great decision to allow your son to be trained early in life, while the child is still fully flexible and open minded. He has a weapon for life, a set of knowledge and a sense of self control that his part of him.