dc 4y puking every half hour since midnight

[mousyfledermaus]

when is it a case for the gp?
I let him have a drink of electrolytic sports drink every 10 min or so.
he's happy enough but weak, slightly warmer than normal (no fever), headache.
didn't have a wee since before bed.

Give him drink of about 5mm (if you've got a syringe it's easier) every 5 minutes. If you have some dirolite (?) it would probably be better.
That's all I had to do with ds when we've been sent to A&E with that. I'd only go to the GP if there are other complications from what you describe.