The good thing is that the others were shocked too. I can imagine that he felt not very comfortable with all the others around him. He made a fool of himself and the group will judge him on his behaviour.
Please allow yourself another view. Most people have no idea what we are doing in the background. Being subject of very bad jokes could also show that we are perceived as normal human beings. They know that we can not eat sugar but it seems odd because we live a very normal life. I mean no one would make a joke in front of someone with a wheelchair. But we diabetics are so 99% normal and sometimes we eat cake and sugar too. Very confusing for those who have no clue about it.
Please do not pick this one jerk and mix it up with other bad thoughts about yourself. I was 17 when I got diabetes and I thought that all girls (especially those I liked) would dislike me very much. I thought that everyone would immediately know that I am different. That I would have diabetic children and so on. So just be fair with yourself.