I've had a many experiences with uncut guys (and I'm uncut), but I have generally assumed they could roll back as easily as I can. This is not always the case, and you shouldn't pull it back any further than he is comfortable with.
If you are taking his foreskin further than it has been before, then he probably has a tight foreskin, and it might get caught on the way back.
If it gets caught, then in my experience, he will feel faint and you should sit or preferably lie him down. If he doesn't get better in a couple of minutes, then call 911 (or equivalent in your country) and have them save his penis.
My 22 year old uncut buddy was a little faint, and recovered after a brief period sitting on the toilet. I understand that is'nt everyones experience, and if it doesn't go back and roll down, then the emergency room may be needed to save the penis (if you pass out and it stays in that state all night, you might lose the penis).
