I'm not familiar with PEGs, is he able to have food as well as the formula? He may be able to have something that is soothing and cool in the mouth. I found rockmelon very good, it didn't have the sugar or acid of other fruits. I tried all sorts of foods when the taste was gone. I've been having weirder cravings than a pregnant woman as it returns.

A humidifier helps moisten the tongue and throat, if you're not using one already.

Its a tough road and there is a way to go yet but you are doing an amazing job!