What Causes Dizziness And Slurred Speech?

My son is experiencing dizziness and some slurred speech. In the last month he had hip surgery and contracted pneumonia. The pneumonia appears to have mostly cleared up as his breathing has returned to normal after antibiotics. 2 days ago he woke up with dizziness and slurred speech and some breathing issues. We took him to the ER, they did a XRay on the Chest, a CT and an MRI on his head, blood work and urine test. All were negative. After a day in observation her felt normal and went home after a clean neuro test. He was fine yesterday, and he woke up this morning ok, however after an afternoon nap he work up with the dizziness and slurred speech again. Yesterday the ER and the hospital doctors had no idea what to do and sent him home for follow up with a doctor since he was normal. The question is what kind of doctor? We are at a loss and don t know what to do next. ER again doesn t seem to make sense.