My sympathies to you, it sounds as if your son (and probably yourself) have been miserable with this. Although infectious mononucleosis occurs commonly, it can still be a real burden to deal with both for the patient and the care-giver.
Your description of your son's symptoms sounds largely consistent with mono. The fever can persist (with some fluctuations) for up to three weeks, and a decreased appetite and lack of energy are certainly consistent as well. I am a bit concerned, however, by your description of the lesions in his mouth and now on his lips. Infectious mononucleosis can often involve a red, extremely painful sore throat and this can also involve the tonsils and the soft, upper palate but I do not believe that it involves ulcers beyond those areas. Sores spreading beyond the confines of the mouth and onto the lips and face is also inconsistent with a diagnosis of mono, to my understanding.
As such, I would recommend another visit to the pediatrician or the emergency room if necessary. Reassessment is prudent at this point. Although mono can be a miserable diagnosis to live with, your description of the lesions inside and outside of his mouth raises some concern. So have him evaluated again and I believe that your concern is appropriate.
I hope that I have been of some help, and take care
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