I would do several things, as you already have had a vet (who makes house calls!)
I would slowly switch him over to canned food for awhile, and I would make the canned food a brothy type, using either warm water, or cooked (and cooled chicken broth) to make the gruel. I would also introduce him slowly to canned pumpkin in this guel. Or slowly add plain yogurt with active cultures to his food.
I would also either elevate his food bowl, or buy an elevated feeder. Cats are gulpers, and elevating their food bowl forces them to slow down the amount of food they are consuming and relax their throat.
I would give him a nice planter of cat grass to chew, if he has stomach upset he will eat the grass prior to eating his meals and feel better.
And I would take a fecal in and check him for worms (if he hasn't been checked already)
If he continues to vomit you want to take him to a cat specialist and have tests run. Cats that vomit regularly are in danger of becoming either anemic or dehydrated.
You can check his hydration, by lightly pinching the scruff of his neck and holding for just a few seconds, then release, watch and see if the scruff goes back down in under 3 seconds and looks normal, if it doesn't he is dehydrated and needs subcu liquids quickly! You can also check the coloration of this gums. If they are pink and happy he is okay, if you press on them, and see a small imprint of a pale pink and then it goes back to normal colors he is fine. If it stays white, or you notice his gums are pale or streaked with white get him to a vet!
Also think about investing in a Drinkwell water fountain for him to encourage him to drink more water.