You might review this question in detail with your diabetes team. He may do better with small doses of Humalog or NovoLog pre-meals rather than the NPH. But, this is more work and obviously more injections. If he is in a kind of honeymoon phase, unfortunately, this will not last for a very long time. You are correct to try to fix the “post-prandial” high levels, but this is a difficult chore even with our current excellent insulin preparations.
The behavior problems could be related to swings in his blood glucose levels, but also could be related to his just not liking having diabetes any more and coming to grips with this is a difficult task for parents as well as siblings and the child/person with the diabetes. You may want to state the obvious: this must be difficult to accept and see what he says. Or, you may also get some assistance form your diabetes team since they have lots of experience with such issues and either have or can refer you to social workers, psychologists, etc. who can assist.
SB