My sixteen year old son (plays baseball and piano) has severe weakness in the fingers of his right hand. mild tremors, muscle atrophy. ideas?

See neurologist: If there is atrophy as you put it it should be checked by a neurologist

Don't delay: Those symptoms generally suggest a neuromuscular problem (with myriad possible causes/diagnoses) and should be evaluated right away. See your pediatrician, who can examine your son and who will likely refer for a consultation with a pediatric neurologist.

Consult neurologist: Your son is having neurological problem according to description ,the disease might have been for sometime . It may or may not be related tonight elbow surgery. Any way ,your son may stop playing baseball and piano and see the neurologist at the earliest to get his opinion, he may do investigation of his right hand and suggest treatment accordingly.