If it is not the constipation, it could very well be that he isn't tolerating something in the food. We too had to try multiple liquid nutritionals before we found one that didn't cause the vomiting. I also have to agree with Eric S on the usefulness of the MJ, it is very easy to get a license and there are dispensaries on almost every corner here in Washington. The MJ was the only thing that helped my husband at the point you are. Gotta get those bowels moving though, lots of water and we used to push mylanta through his tube too.
My heartfelt support to you.
Jennie