So that may be part of the problem -- the fats slow down the digestive process. Do call his doctor and ask for help.
Type 1 diabetes is not a disease whereby the doctor can automatically say, "you need x amount of insulin" -- each person's dosage must be tailored exactly to that particular person, considering their activity, metabolism, and diet. So it probably just isn't quite right yet. Don't lose heart. Adjustments will probably solve the problem.