This problem is not as simple as it may first seem. It looks like your child is just too thin-skinned and unable to handle the give-and-take of being ten or thirteen. It is a rough time and getting his feelings hurt happens easily if he lets it. This is just a fact of life for children, and for adults too, for that matter. Your encouraging him to be a little tougher is normally good advice; but the sign may be a symptom of other problems.
Children with learning problems are more likely to get their feelings hurt by teachers and by other children. It hurts when people call attention to your youngster's not doing well, especially if he has tried to do well. Children whose self-esteem is already low feel even worse when things happen other youngsters do not think much about one way or the other. Children who have high stress in their lives are much more vulnerable than those who do not. Handicapped youngsters, children with other physical or emotional problems, and those who are already self-conscious for some reason get their feelings hurt easily; and just being a child brings its own vulnerabilities. It really can be complicated for children.
It is not enough to simply encourage your child to be more thick-skinned. He needs help with his feelings and with how he thinks about himself. You can help here better than anyone since you are likely to be present when his feelings are hurting.
At first, take your child's side. You can start by getting a little angry for him about what happened and with whomever did it to him. He matters; and his feelings matter to you. He needs you on his side right now. Your advice and guidance can come later.