Hi Dot,
Standard advice at our clinic is that more than two vomits in a couple of hours requires a phone call to the registrar in the children's ward and probable admission. This is firstly because keeping the BS stable when a child can't keep anything down is very difficult, and secondly, dehydration is an especially serious danger to Type 1's. You need to keep a very close check on ketones. Often these will rise when the child is sick, meaning they probably need more insulin, but as they cannot keep anything down, you cannot treat hypos, so it becomes a vicious circle. They also need their blood gases checking.
DKA is a serious danger during vomiting bugs. At hospital they can be given intravenous glucose and insulin to stabilise things till the vomiting stops. So if he throws up more than a couple of times, ring the hospital, and keep checking the ketones, even if his BS is low.
Hope this helps.
Best wishes
Sue