After 20 years with type 1 I'm still struggling with the unpredictability of it all!
One answer could be to inject again, if he is ok with that. That's what I'd do if I ate my meal and then was hungry again. I'd decide what to eat next, calculate the carbs in it (after 20 years, I do that roughly in my head) and then have another jab. If still hungry, do it again.
Also if he always injects specifically for pud then it doesn't matter if he decides after the main course that he doesn't want any pudding. The original injection was only for the main course.
The things you have to be aware of/take into account:
- if he hates injections or really doesn't want another jab, this is not going to be a good solution for obvious reasons. I can remember as a teen suddenly 'cracking' and not wanting to take any more injections than the bare minimum (in fact, I stopped taking any for a while). It's about realising that it gives you more control... you have to come to that realisation in your own time really.
- like everything to do with diabetes, it's not straightforward in terms of 'eat x, inject x, woopee, perfectly balanced'. The combinations of food and the build up of insulin will all interact with each other. Eg eating a relatively sweet pudding after a meal will result in a slower blood glucose 'spike' than eating something sweet by itself or before a meal. But that is no different really to the way you work out everything with diabetes; trial and error.
good luck, hope it helps. In many ways it should help him (and you) learn more/faster about the way insulin and food interacts in his body - some really good info and a good foundation to set him up for life with diabetes.