My 15 month old baby boy is constantly being sick. Help?

My little boy has been sick now for 24 hours. He had diarrhoea this morning and sickness, but is just being sick now. He cannot keep anything down. I have just been giving him water all day but even that comes back up. He has a slight temperature. There is this bug going round our area. What should I do to try and make it more bearable for him. Calpol won't stay down. He is very lethargic and is sleeping most of the time. He is our first child so we haven't any past experience to go by. Many thanks for any support or suggestions.