I suggest you get your child into swimming. The aggression can be used up in the pool. My nephew 15 (not diabetic) has been attending a swimming club for 2 years and since then I've never seen him argue with his Mother or disrespect her. He is more thoughtful and caring, too. He has also joined a group where he plays the drums so again his energy is used elsewhere.
Do you and the other lady live close to each other? If you do maybe you can meet up so your children can join in the same activity? A bonding between them would also make them aware that they are not the only child that has the problem. They're probably going through a stage of "Why me?" The reason I know that is because I had a different illness when I was 10 and I couldn't play with friends or my younger sisters because the physical results caused my eyes to swell. I had to ask one of my sister's to lead me back to our caravan and when she got inside she told our Grandparents I was blind. She was only 5 years old and didn't realise that if I put a cold wet flannel on my face and rested for an hour I'd be able to see again.
I had problems at school too because my body looked like I had measles and nettle rash. The children used to call me "Monster" but I used to ignore them because I knew eventually it would go away again. At secondary school I was regularly in the Nurse's room getting covered in calamine lotion. She never wanted to put it on my face, however, I always told her to because I'd rather look stupid than itch all the time.
Does your child inject himself in front of the other children? Maybe it's time for the school to make all the children diabetes aware? Don't forget from being small we always keep children away from sharp items, e.g. scissors, etc., so they'll think your child is doing something wrong and possibly don't want to play with him.
I also thought of things like martial arts, etc., but that would make them worse because they'd both know how to hurt people more.
Is there a childrens support group where they can go and play together? I think the more children like him the better he'll feel. Unfortunately, my problem was very rare.
I hope this insight has helped you.