[cypher]

If he's already eating lots of nuts & seeds then he doesn't really need more of the same, have you tried some extra protein foods like plain cooked chicken, boiled or scrambled egg or tofu? Plain dry oats are a good all round food too.
His main mix should be giving him the nutrients he needs really as long as he is eating it & not being picky & if he's still having veg that will help generally too so it may just be that he will gain more weight when his health improves.