I had a pom who suffered with colitis...she had blood and mucus in her stools, but never threw up.
Horrible question, but have you had a good lot at the vomit? worms can make pups sick, how often has he been wormed?
Back on the colitis....we gave Lola boiled chicken and rice in small amounts when she had colitis, and it always sorted it, then went back to her normal food gradually and in small amounts. We had a GSD with colitis too, and our vet said feed him on tinned Chappie, the original yucky smelling fishy sort. She said when they had dogs with digestive problems in at the vet hospitals, that was what they always fed them. He was fine with Chappie, but if you tried anything else, his colitis came back, even the complete foods which were meant to be for sensitive stomachs.
Hope he soon clears up!
(and yes, I do think some vets prescribe special foods just to get more money out of you, but not all of them, and not all the time.)