What Causes Bad Headaches With Dizziness & Nausea?

Hi, My son is 24 yrs old. for 3 weeks now he has been having really bad headaches with vomiting dizziness. he's not getting any better. he has been to the er 3 times already. white blood cells were up alittle with bands around them. they did a spinal tap, took blood, ct scan, all came back fine. he does have High blood pressure and is on meds for it. when he is laying down he is ok, but when he gets up or sits up head starts to pound really bad and gets sick to his stomach. this had been going on to long now. would you know why he is feeling like this? please help thanks