Although the taste of baking soda and salt is not appetizing, it is really one of the best oral rinses a person can have. However, you can down the road request what is known as Magic Mouthwash, a prescription rinse with Lidocaine(sp?). Several patients have had comfort from this.
Personally I found sipping on ice chips and water was some relief. Try to get his mind occupied with other things. It will probably get far worse in the coming weeks and he may continue downhill for days after treatment stops. It's not the end of the world but it may feel that way at times. Anti-anxiety meds may also be considered. Courage. We do get through this.