That's always a bummer
Did you happen to get a new container of the supplement close to when he decided he won't eat it anymore? If so, the company has changed something regarding the flavoring that doesn't suit him - lol lol I am laughing because you are not alone in this
Sounds like he is boarded?
Applesauce does work or you could buy those mini bottles of 100% pure apple juice (no sugars added) and mix his feed & supplement with that.
I was going to suggest syringing the supplement down him, if its powder form (doesn't work with granules). But again, it sounds like he's boarded, so syringing is out unless you're there every day.
Carrots and apples chopped in a blender also work but that is something that needs to be kept in a refrigerator. Hope you can get this figured out