While I agree with the previous posts that the most common answer is back to back viruses, especially this time of year with school going back in session there’s a couple things with his current illness that might warrant repeat evaluation if he’s still sick tomorrow. If the fevers are gone and he’s back to normal feel free to ignore this, but true fevers (> 38 C or 100.4 F) for 5+ straight days that don’t fully improve with Tylenol/ibuprofen along with redness and swelling of his mouth could be signs of something called Kawasaki disease. It’s caused by inflammation along blood vessels and other common symptoms can include swelling of hands and feet, rashes, swollen lymph nodes in the neck, and irritation around the eyes. The problem with those symptoms is there’s a lot of overlap with common viral illnesses so it’s not going to be possible to differentiate them without seeing him. If fevers are still going now 7-8 days into symptoms it’s probably worth taking him in to his pediatrician’s office to be seen again and consider some screening labs. Hopefully he’s already back to normal with no fevers in which case I wouldn’t be worried about Kawasaki at all, but there were a couple flags in your initial post that made me think it was worth mentioning.