For his weight loss even though he is eating (cachexia/wasting syndrome) try 3 capsules of concentrated Omega 3 fish oil pills.
You can look up info on it on this site, or on internet. My friend started doing the same thing with his GIST and lost 15 pounds in a week. He started gaining muscle back-not fat or water-within 2 days of starting the pills.
I'm not making a guarantee, maybe we were just lucky, but it sure is worth a try.
If he's been nauseated and vomiting a lot, they should be giving him anti-nausea meds and also hydrating him with potassium IVs? Try to also give him protein shakes to get the nutrition in him and keep the muscle mass stable. Spirutein has some good shake mixes in many flavors at the health food stores. Its normally used for weight loss, but if you put in 2 scoops to a 16 oz glass of lowfat milk it will be a weight gainer. That is only if you can get a handle on the cachexia.
There are two now chemos coming out for GIST, sunatinib malate SU11248(Sutent), and BMS 354825. Pfizer's Sutent is already submitted to the FDA for approval and may come out in 2006, but for right now they are giving it out for compassionate measure at some of its old clinical test sites. I think you can contact Pfizer to find out where, but otherwise it should be listed in the clinical trials lists on some of the Gist info sites.
I hope the best for you.