Hello, Welcome to Healthcare Magic forum. History of fever and knee pain may point to an underlying chronic bacterial infection( osteomyelitis ) most likely but can also occur in chronic inflammatory disorders like autoimmune diseases or malignancy or viral infections.The weight may be apropriate for stature but more important to notice is any recent weight loss or failure to gain weight for many months. Since there is painful limitation in the range of movement but no bony changes on xrays you need to rule out other musculoskeletal or systemic or infective problems.This requires a step by step evaluation first by physical examination of the child by a pediatrician or pediatric orthopaedist and depending on which further investigation may be done like ultrasound or mri of the affected limb, ultrasound abdomen and pelvis. blood tests like complete blood counts, esr, crp, bleeding profile etc Since there is a strong history of malignancy in family, it is better to be investigated and be on the safer side, however it s less likely as most bone tumours should be visible on xrays. Wishing him a speedy recovery. Regards