It would help if you could describe the vomiting a little more (I
know, it's sort of gross). When does he vomit and what is in the
vomit?

I agree with jmc in saying that I don't trust vet-recommended food. If
you look at the ingredients, they are downright scary. There are
probably grains in that food too, which can sometimes be an allergen
for cats. IMO you should feed a high-quality, grain free wet food. It
won't cost any more than the prescription food. Try a novel protein
like duck, venison, or quail to see if it helps. No fish, as that can
be an allergen too.

Warm the food a bit. Sometimes cold food can cause a cat to vomit, and
at age 16, it will make the food more appealing to his diminished
sense of smell.

I know how frustrating this is. Vomiting is one of those catch-all
things that can be difficult to diagnose.