We just survived our 2nd bout of this! (Yes, they can get it twice.) At 3 1/2 they did give him magic mouthwash...the big deal is whether he understands to spit and not swallow the medicine. We had our son practice with water first. The other thing I learned was that there are lots of different types of "magic mouthwash" depending on the doctor.
We also did the alternating ibuprofen/acetaminophin (tylenol).
For food, we did lots of popsicles (nothing with citrus), yogurt, jello, and pudding. The goal is to keep them from getting dehydrated. (Worry about nutrition after they get better.)
I would definitely call back and ask for something else to help the pain. While it has to run its course, you should be able to give him something to help him manage the pain.