The 4 yr old (non CAH) son of a friend of ours just had this after Christmas: he was quite sick with it - lots of sores in and around his mouth, as well as a fever. I think that I would tend to err on the side of caution and double (triple??) your son up, especially if he’s so uncomfortable. I seem to remember that Tylenol was all that helped our friends’ son ... and also remember they went through a lot of popscicles! Praying that he’s feeling better soon.