Hi Jaxx,
It must be so stressful for you for so many reasons, especially ensuring your son eats well enough. I hope you're getting plenty of support from his hospital team. They should be able to advise about diet as they'll monitor your son's growth over the years.
It does take a while to get blood sugar levels under control and I'm sure theyve erred on the side of caution with such a young child with the insulin doses.
Have you looked at the Parents section here for support?