My 20 year old son has for the past week been having severe body shakes, I have had him tested for drugs and have had additional test done to rule out blood clots in his lungs. What else can cause the shaking?

What do the doctors say? It can be many things from mercury poisoning to neorological problem to Parkinson's to MS to brain tumor to alcohol withdrawal, drug abuse , head injury - he should be hospitalized and tested thouroughly. If your doctor isn't following through, go to a quality neurologist.