Take some little toys for him to play with and let him dictate the hunt. If he wants to leave after an hour leave after an hour. If you force him to stay longer because you want to, he will not have fun and never want to go again. Just keep it lite haerted and make it fun for him. Harvesting an animal is one of the least important agendas of that day. I just took my 4 yo nephew out on thursday for the first time turkey hunting se the post in the bowhunting forum called the best turkey hunt ever. he's now adicted and I took him again yesterday and he's going with me again tonight for the last evening of my season. Good luck and it's amazing to see the wonders of the outdoors played out in the face of a child.
Good huntin,
Jeff