18 son complains w knee pain, hasn't injured it been sick last while stomach issues can this be lukemia ?

Not necessarily at -: All. There isn't much to go on to make a diagnosis like that. Get him see his pcp to be evaluated with an examination and some labs. While any disease is a possibility, its not very probable, as there are more common causes of painful knee in a 18 year old. Do yourself a favor and stop worrying, by getting him into seeing his pcp soon. Good luck.

Unlikely: More likely knee instability. Rheumatologist or orthopedic surgeon should provide insight.