my nephew is sugar-intolerant and has no problem in taking his own fruit with him to school for when the children have snacks.
You can always get diabetic cake (although very expensive) so that he can still have pudding the same as everyone else, allbeit one that his body is ok with.
One comment about some of the postings above - have you ever looked at a pot of yoghurt for sugar content? Unless it is pure natural yoghurt (which is not very interesting to have in place of a pudding) they are all PACKED with sugar - especially the ones for kids.
Look at your local supermarket in the "free" section for sugar-free/diabetic options. Well worth the extra bit of money if it means your son can cope better at school.
Good luck!