prolonged seizure attacks

I have a 19 yr old son who has Prader-Willi syndrome. Unrelated to the syndrome about 1 1/2 year ago he started having seizure like attacks. He would become unresponsive, non-verbal, and have tremors that appeared to be like seizures. However, the attack will go on for 1 to 3 hrs. He then slowly  comes out of it. He has had mutilple EEG's both short term and 24 hr. And even when an attack was going on the EEG's were all negative. He has had MRI, CAT scan, and even a spinal tap everything including blood work came back normal! Only after 4 days in the hospital they found a UTI and he was treated for that. But, it was negative at first. He has had some problems with a blockage in the urethra but that was taken care of shortly after this started. He has had about 10 - 12 attacks like this since then. Nobody seems to know what it could be. I am very scared and frustrated!!! Any ideas what it might be or test that should be performed? Thank you.