This may have been asked and answered, but after speaking to my doggy pal, I wondered if you had tried Homer on wet food? I wondered if it would stay down better. Either tinned or from a tray I mean.
The other thing I was thinking about, was perhaps considering discussing erythromycin antibiotic with your vet. Another prokinetic, we see it used in diabetics with gastric paresis when other drugs have failed.
I think too there are loads of natural substances that can help - ginger and cucumarin are two that are mentioned frequently. Also peppermint, dandelion rooting and artichoke, but no idea about the latter ones and how you would use them.