I know for me there was no single thing that really did the trick. I tried a variety of things at different times. Overall I think the soothing effect of room temperature, slightly flat Club Soda as a gargle gave me some comfort through keeping things moist and clean.

Viscous lidocaine ( 2%, a prescription is needed) helped to numb things when the irritation got really bad. You just need to be a bit careful not to use too much, and to avoid swallowing it. It numbs your mouth, but it'll also freeze your throat which is almost as unpleasant as having it sore
Rest is best. He's going to get increasingly fatigued, and the more he sleeps the easier he'll find things
Wayne