You didn't mention what happened before the "episodes". When my first daughter was old enough to walk (sorry, I don't remember how old she was the first time, but she started walking at 10 months) she would do a silent cry until she passed out if she was startled and hurt at the same time. So, for instance, she tripped and hit her head on the sidewalk. It surprised her and hurt, so she did that first big cry that all babies do, but she wouldn't take a breath and continue sobbing. She would turn purple, her eyes would roll back into her head, and she would pass out, then turn completely white and clammy. Talk about scary for first time parents who were 20 and 21 years old!! She did this once in a while, when conditions were right, the last one when she was 4 years old. She's a completely fine, healthy, intelligent 7 1/2 year old now.
Follow up with your doctor, get a second opinion if you feel you need one, but if nothing is found just relax and pray that he grows out of it.