What Causes Fainting In The Night?

My 61 year old husband visited the GP because he has had three episodes of fainting in the past six months, all occuring in the middle of the night upon rising to go to the bathroom. He has naturally low blood pressure and feels lightheaded during the day sometimes if he stands too quickly. He has also lost weight even though he eats well. The doctor took bloodwork and an EKG. The EKG showed a healthy heart, but his white blood cell and platelets are low. He is referred to a hematologist, which is producing anxiety. The appointment is not for a week and a half and the GP is on vacation so not available to answer questions. Feedback on what might be going on with my husband s health?