This used to happen with my difficult child when he was in a mainstream primary school. Other children would lie in wait for him because he was an easy target. Then, because of his short-term memory and verbal processing difficulties, he wouldn't be able to explain coherently what happened to him. Or, he would be tormented to the point where he would be in a full-on tantrum by the time the teachers or recess monitors noticed that anything was happening. Then the teachers would pin blame on difficult child because that was the easy path.
My SO solved this by going to the school at recess time with a video camera and filming the children in the school yard picking on our difficult child and inciting him into a rage. With direct evidence in front of them, the school had no choice but to start disciplining the other children.
Your husband is right...shadowing your difficult child on his walks is a good idea, especially if you're in a position to film or take pictures of any incidents that might happen. You'll be right there to protect your son, and you will have something concrete if you need to confront parents, or talk to the authorities about what's going on.
Bigger and older doesn't necessarily help, either. At 17 and 6 ft 4, our difficult child still gets tormented by smaller, younger children, one of the many reasons we don't let him go anywhere unattended. He is with one of us 24 hours a day.
Best of luck,
Trinity
