Hi Susie
Obviously I don't know how things went for other T1s after diagnosis, but my teenage son who was diagnosed just over a year ago had his first hypo 5 weeks in. His first one wasn't a true hypo figures wise - it was 4.1 - but he went deathly white, got the shakes, felt faint and nauseous, but the coke worked like magic. Trust you have some, or lucozade, or dextrose tablets etc in.
I would suggest you try and gently explain to him that they will happen from time to time (each time it was a signal to my son's team to lower his Lantus) and that is all the more reason to test regularly. Tell him not to ignore things if he feels a bit 'odd', don't be afraid to either test in the classroom or ask to leave (with a friend) but don't ignore the signs. Also reassure him he will be fine once he treats it. I had visions of my son dropping to the floor at the first sign - no-one explained really what was likely to happen, so I think we all lived in fear. I never asked my son what he expected either, but I think he was scared witless by the thought of it. It's not nice, but usually things perk up pretty quickly. Once you have all been through the first one or two you will relax a little about it. It will unnerve him (and you) to begin with, but you soon all become expert at noticing the signs. My son's colour is usually the first sign of trouble.
My son had hypos 3 or 4 days in a row, so the Lantus was lowered, he stabilized for a week or two, then they started regularly again, so it was lowered again, and that's how it goes. Others will tell you on here thought that there is no rhyme nor reason to it, and just when you think things are ticking along nicely, a hypo can strike. So just make sure he carries coke/lucozade/dextrose etc around with him AT ALL TIMES.
Hope that takes some of the mystery away for you, and also I haven't scared you witless!
Tina