Child Has Headache When Bending Over, Urinating, Breathing Deeply. CT Scan Negative. Prescribed Propranolol

My 15 year old son is experiencing what appears to be thunderclap headaches. Triggers include bending over, urinating, breathing deeply. The headache hits full blast quickly, his BP hits around 175/125. We've taken him to the hospital and they performed a cat scan, spinal tap, and that came back negative. The headache lessens over a few hours and seems to respond well to Tylenol. We've seen a neurologist and he ordered MRI and MRA, both came back negative. We are waiting on a 24 hour urine test. We also saw a pediatric cardiologist and the ekg and heart sonogram, both came back negative. The first Neurologist said it may be RCVS. A second Neuro said it was a migrain and has prescribed Propranolol 10mg once a day.