One of our trips, when we stayed at the Poly, my youngest we got sick the day before we were supposed to fly home. We got onto the monorail to go to Epcot and he was fine, half-way there he said he was tired. By the time we got through the gates and into the first shop on the left side he was burning up with fever and so tired and weak he could hardly walk. Of course we weren't prepared. Luckily they had single packs of children's ibuprophen and Tylenol for sale. I gave him a dose and he and I went back to the room. My husband and other son went on to eat and play for a while. I got him back onto the monorail. He slept the whole way back on the monorail. Luckily we were staying at the Poly because I could just take a short walk carrying him back to the room. We alternated Tylenol and ibuprophen every 3 hours until fever broke and he felt a bit better. By the next day he was still sick, but could do a bit more. We kept our flight and things just got better from there as far as he felt. I was glad it was our last full day there. I am not sure what I would have done in the situation that you are in, call the doctor and see their opinion. Maybe a few days on an antibiotic would help out? Ya know, when I was little the doctors gave a shot to get kids over illness quickly. Too bad that isn't what they do anymore.
I wish you luck and good wishes that he recovers quickly and the trip is perfect.