Sorry to hear about your husband's IBS diagnosis. It's great that you're looking up information for him and being so supportive!
If you click on the IBS Diet & Recipes link to the left, you should be able to get to the IBS Diet FAQ. The weight-gain question is pretty common and Heather advises on how to maintain or gain weight with IBS. Her tips include incorporating high-calorie quick breads (sugar is fine for IBSers because it doesn't stimulate or irritate the digestive tract - I don't know if your husband has other health issues that require him to be on a low-sugar diet?) and lifting weights. There are a lot of safe bread recipes in the recipe index (which you can also get to by clicking on the IBS Diet & Recipes link).
I'm also trying to gain weight. I've been supplementing my meals with Luna Bars (which are safe for most people, although the inulin in them can cause gas for IBSers), smoothies, and trying to have dessert more often. So far it seems to be working.
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Re: weight loss
It's great that you're looking up information for him and being so supportive! 
So far it seems to be working.
