S.E.
This is very normal. Their taste buds change quickly at this point and their likes and dislike change in the blink of an eye. Don't start catering now, he will out grow it and if you take the 'anything to get him to eat' road, you will be stuck eating nothing but pancakes and mac-n-cheese till he's 12. Keep offering him a variety of foods and keep requesting that he at least try it. But don't push or make an argument about it (he will dig in his heels at that point). It will get better, you just have to tough it out for a while. The stomach bug may also have a lot to do with his recent change in taste.
P.S. His thinness is more likely due to his growth and development and being sick at this point than his picky eating, I wouldn't worry about it.