If i were you, I would keep your son with a helmet on for his safety and for the getting picked on part, dont worry too much becuase they are still only 11 and immature. If it gets really serious and makes your son really upset and stuff, I'd go talk to his teacher about it and the parents of the people who are picking on your son, and you should ask your sons teacher to talk to them why wearing a helmet while riding a bike is important!