Glibenclamide is a sulfonylurea which causes your pancreas' beta cells to work harder and therefore produce more insulin.
Now, sulfonylureas are a very common type of drug in treating type 2 (and type 1.5), but they're usually prescribed before insulin rather than after. Furthermore, there are drugs like Gliclazide which are widely considered to be less harmful long-term.
If I were in your shoes, I would ask the doctor why he feels a sulfonylurea would benefit your son rather than to increase his insulin doses. It is possible that there may be another reason, but if it's only to increase his total insulin output, I'd avoid that drug like the plague if I were him.