It will normally take up to two weeks to get over the flu totally, but the worst should be over within a week.
If you son has a very high fever and a very sore throat, then I would strongly suggest that you give him homeopathic Belladonna in a 6 or 30c potency. It is not harmful at all and you can get this from a healthfood store and just give him 3 tablets a day dissolved under the tongue for two to three days and see if this helps the symptoms.
Please try this, as it is certainly better than giving Tylenol. Also, a fever happens for a reason and that is to burn off pyrogens which are fever causing agents and should not be interfered with. It is all part of the natural healing process and it will reduce on its own without intervention.
Please try and Belladonna though as it definitely sounds like the indicated remedy to me.