When my DD's toenail was found to be suddenly hanging off (she was about 18 months) I called our GP surgery and was told it had to be A&E - I'm not sure if that's just because she was so little so probably best to ring your surgery to check.
She wasn't in any pain though - when she was examined it transpired there was a little sliver of new nail growing underneath so the doctor just snipped the nail off and dressed it.
A&E... he won't get in to see a GP quickly anyway so you might as well take him to hospital. Toenails really are very painful, if his has been partially ripped off he runs the risk of infection. Take him now.
Well he will be screaming.. it bloody hurts!!! I ripped two of my fingernails off a couple of years ago, and I was in agony for at least two weeks.. and I really do mean the sort of pain that makes you feel sick.
You can wash the toe/nail and then wrap a plaster or steri strips around it, and there is a chance that the nail may re-bed itself if you do it quickly enough.. but it is more likely that the nail will fall off in a day or so anyway.
Put him in sandals, no socks, lots of fresh air to the area, and tell him to try his best not to catch it on anything for as long as possible as it will be very painful.
A & E wont do anything, other than x-ray the foot to check nothing is broken, so if you are sure that isnt the case, you would be wasting your day going to A & E.