A couple of thoughts -
The cold and snacks might be getting in the way of his appetite.
Be sure that the snacks you offer are foods that he ought to be eating, and foods that won't get in the way of an appetite (a handful of grapes is a lot less filling than a banana, either is preferable to a twinkie).
Or for a more stringent, no nonsense, lots of tears now but perhaps a permanent fix later, you can try my MIL's old school approach.
My MIL infamously decided that she wasn't going to be a short order chef. She offered the same scrambled egg (that he had asked for, and then refused) to my husband for 2+ days until he relented and ate it. From that point forward, he ate everything set before him without challenge.
Good luck to you and yours,
F. B.