I use only powdered natural tooth paste. If ur interested or if anyone is interested I willing get the name for you.

Try encouraging him to drink water and wash his face, especially lips, that's always solved it for me and Im severely allergic.
Also always have antihistamines!

Check if you are allergic to Salicylates which are in so many foods!

Allergens can stay in the saliva for as long as 24 hrs so thats something to consider, i suffer from a sulphite allergy which is really tricky to deal with, i react when kissing my husband i understand how scary it is.

The high amount of alcohol probably breaks down the proteins vs the small amount in mouthwash. I've read proteins take a while to break down in the mouth and that eating something else helps wipe out whatever was previously eaten.
My daughter has so many food allergies, I'm terrified for when she starts dating =/