Possible Gastro? Or some other infection?

DS, 20 months, was very sick on sunday night. He'd had a bit of Diarrhoea on friday, saturday and sunday mornings, he's been really gassy and his lower tummy has been very sore.
On sunday night he threw up everything he'd eaten and drunk that day (was rather nasty, and he'd drunk well over a litre of water), and had really nasty and acidic Diarrhoea.
Yesterday he was ok, no tummy pains, all happy, didn't eat much and we kept what he did eat as bland things (plain rice cakes etc), bu was otherwhise ok.
Today he still hasn't eaten much, and yet is quite hungry, but his Diarrhoea has come back again and his tummy is quite tender. He has lots of gas again too.

Hubby doesn't think it's bad enough to take him to the doctor, especially as he is still taking fluids, but he's been sick for almost 5 days now and I don't know what to do to help him feel better.

Any advice would be greatly appreciated.

I would take him to the Doctor- even if it is only to ease your mind.

I would also be on the lookout for dehydration in the meantime- even though he is taking fluids, dehydration can occur quickly when diahrroea is occurring...