11yr Old Son With Low Grade Fevers Almost Every Evening

11yr old son with low grade fevers almost every evening for a little over a month w/ headaches off /on. Fevers are not present in the morning . He has some mouth sores that we recently found . Just very recently (today) the back of his knees/ upper calf just started to ache - even to the touch . this just started today. No joint pain throughout the past month. His appetite hasn't been the best , but he has been eating . No vomiting . Some abdominal pain, but nothing severe.