Gliclazide stimulates the pancreas to produce more insulin.
So this in turn could be causing the hypos.
I would contact his doctor (and also talk to the hospital - he should be under the care of the specialist diabetes team) to discuss perhaps coming off or at least reducing the Gliclazide. If he is suffering from hypos this is an indication that he may now be producing too much insulin.
To be put on insulin at first diagnosis suggests that his Blood Glucose must have been quite high at the time.
I assume you were testing his BG in the middle of the night before giving him the lucozade and biscuits; what was it? I assume he was having the symptoms of a hypo, but it may just have been a false hypo if he has been running at a high BG for a long time before diagnosis.
It requires further tests to confirm the diagnosis, but I would be surprised if he does not have some problem related to BG control. There are plenty of posters here who have been mis-diagnosed as to the specific type of diabetes (or in some cases reactive hypoglycemia) but the first step of diagnosis is a blood test with abnormally high BG. I assume that this is what happened.