Shut up with the bots - I had enough of the last lot of worms you had me creeped out with!!!!
If the horse is eating other stuff then its mouth must be feeling easier, he might associate his own feed with pain or it could just be that you have somehow got a bag of contaminated feed - it can happen
I have had horses with really sore mouths from injuries - my old mare even managed to bite her own tongue really badly but she they stopped eating even if they did make a mucky mess of trying!!!