ds 3y sick - when to worry about dehydration

[mousymouse]

ds has been sick since last night, didn*t have much of an apetite for dinner but ate some. started vomiting 2 hours later and from then on every hour or two hours.
cant keep anything in. anything he has comes back up again reeking strongly of bile. went for a wee twice since it started. I am offering a few zips of gatorade and water but the little hes having comes up again...
poor lad is sitting on the sofa listening to stories and is very weak, understanably.

thank you so much for your answer.
I am giving him gatorade, the isotonic version non fizzy, zip by zip in a baby zippy cup. he has one zip and I put the timer on 10 for him to have the next zip or he would down the whole cup...
his mouth is def dry (and smelly) but he is in reasonable good mood listening to the carneval of animals at the moment.

my DD aged 3 did end up on a drip but I think what made it so bad was heat (we were away at time) and very watery, constant diarrohea. she became pretty non responsive, just wanted to curl up on the bed. agree with pixie on signs to look for and constant small sips being the best idea.

thank you both.
wish me luck, no puking for two hours now...moved on to peter and the wolf now - 3 different versions...

If worried get some rehydration medicine such as Dioralyte (worth having in the house tbh anyway, as my DS was very sick for days once when DH was away and I had trouble getting to the pharmacy).