You could get some L-Lysine powder from a health food store and give him 250mg twice a day in some canned food to start, up to 500mg twice a day for a few days until symptoms subside. Also hanging out with him in the bathroom while a hot shower is running might help him to breath better.
Please don't force-feed him any kibble. He could choke. If he's eating canned food, that's great, that's what he should be eating anyway. Let him eat as much as he wants, maybe try a few different flavours to keep him interested. You could stir in some warm water to make it smell stronger, and add smelly things like low-sodium tuna or salmon juice, or some sardines or dried bonito flakes (aka
Kitty Kaviar). What type of canned is it that he eats?