What are the most likely causes for a new onset of low platelet count in a elderly man who takes meds for atrial fibrillation, gerd, and depression?

It depends. There are so many possibilities that it's best to consult a physician. Any answer you receive on this site may be misleading, since the responder won't have the patient's entire history to put into context. I hope this answer helps.
Few causes. It could be simple causes such medications, immune related platelet destruction (itp) or more complicated issues like bone marrow problems such as myelodysplasia.