As a person who has dealt with stomach and intestinal ailments, I must say that this character will need a strict diet by which to continue living! At least until you find a cure. And if he recovers, he may find he has lost the ability to tolerate some foods, say, like a wheat sensitivity or something. But I think that would be intersting. If something that drastic happened to me, I'd mourn the loss of those foods for a while, and then my tastes would change to accomodate the foods I can tolerate. Maybe this will affect him in other ways, too, like how he views the world. Or maybe he'll be able to read personalities by the ways and things that people eat.

For a cure:
Take a moment and remind yourself that you're writing fiction. Then take ten minutes and look up some treatments for gastroparesis; even if the options are limited, you can use science as a basis to concoct a cure. There are many herbs and natural healing aids that my doctors were willing to discuss with me, so feel free to use those sources, too.

Just a few things I know of:

Swedish bitters - used to stimulate the stomach and prepare it for the reception of food.

Ginger/liquorice root/calendula - separately, or in a tea, used to calm the stomach.

Garlic - to fight off parasites and stimulate the immune system.

Honey and cider vinegar - used to tame stomach acid.

Turmeric - used to treat inflamation of any kind.

Anyhow, I hope this helps!