I know exactly how you feel my boy has been in and out of the vets since Saturday, they gave him antibiotics for his temprature just over 39 degrees, and then gave him a shot for the vomiting, then he developed diarea and has been on scour ban since.
He is only just coming good now!
How did we do it - well my vets are wonderful, me not so good, he was off all food and (water (limited) for 24 hours and just given scourban. This placed a lining back on his stomach and then he was allowed small amounts of food, when I was told small they meant small (broke my heart given he is such a hungary boy) and I was told to boil some rice with meat or chicken add some cottage cheese and some natural yoghart. I fed him small amounts after the 24 hours. Small amounts means around a table spoon at say 6pm then another at say 8pm.
That was it for the first night - he held this down with his water small amounts of water, the vet said either an ice cube or around 10-15 licks from the bowl.
He is only coming good now and he will have his first full meal on Sunday, with the amounts increasing more and more each time.
I know how you feel its very hard to sit back and wonder what is wrong, scary too.
Hope that this helps