Signs of concussion include vomiting, confusion, bruising behind ears/eyes.

It's ok to let him sleep, just check on him every now and again, try to rouse him occasionally- he doesn't have to wake, just stir.

Go with your gut. If you think something is wrong take him to see a doctor/ hospital/ call nurse on call.
It is better to go and find out he's fine, than not and have a very sick little boy.