You need to monitor his wet nappies. The first signs of dehydration you will see are a reduction in the number of wees he is doing. The other symptoms - listlessness, dry skin, dry mouth, etc. - come later.

Babies with diarrhoea don't tend to dehydrate as quickly, though, as babies with vomiting. Make sure you keep his fluids up - plenty of formula feeds. If you become worries, you can also feed him Gastrolyte, which you can find from a chemist - it is mixed up in water and contains some essential minerals that keep them hydrated if for some reason they are not tolerating formula.

If you are concerned he is becomign dehydrated, then you should go to your nearest emergency department, or to your GP, but only if they can see you straight away.