what will happen has diabetes and not taking his insulin?

His blood sugar (glucose) levels will rise, ma'am.

Higher than 'normal' blood sugar levels make the person MORE LIKELY to go on to develop diabetes-related complications. I won't go into details about the complications as I have no wish to frighten you, but I will say that some complications can be a real pain ... literally; some can prove embarrassing; and some could prove fatal.

Is "he" checking his blood sugar levels, ma'am? Any time they rise above 13.9 mmol/l [that's 250 mg/dL if you are, in fact, one of my American cousins] he should be checking for ketones too. A high level of ketones is indicative that he MAY be on the way to developing diabetic ketoacidosis (DKA), which is what occurs predominantly in those with type 1 diabetes, or hyperglycemic hyperosmolar non-ketotic syndrome (HHNS), though dependent on how severe his diabetes is, could also result in DKA. Both of these conditions can very quickly result in an emergency situation where the sufferer would need to be admitted to hospital to keep him alive.

DKA and HHNS are both very damaging to the body's organs, blood vessels, and nerves, which will bring on the diabetes-related complications on more quickly than if blood sugar levels were simply high, but not high enough to develop either of these conditions.