My Lab poops in the bed of my truck everytime I load him up. How can I help my dog to learn that what he's doing is not acceptable?

My 5 Year Old Lab has a problem with going to the bathroom. A good friend suggested I contact you. I hope you can help.

My Lab poops in the bed of my truck everytime I load him up to take him to the river or for a run in the country. He just gets so excited, I don't think he can help himself. My Lab lives to hunt and retrieve! That's all he's interested in. He doesn't care for petting and playing, just working. He gets a good deal of excercise as he walks three miles with me each morning. He gets fed 4 cups of good dry dog food at 6 am and again at 6 pm. He is a very hyper and intense dog. When we are going hunting or retrieving, we forgo our morning walk cause we don't have time to walk and then go hunting. I try to get him to "go" before he gets in my truck. Usually I am not successful. He goes any way. I never feed him prior to loading him up. He usually hasn't eaten in 12 to 18 hours when he gets in the truck. I have tried loading him up and taking him just down the road and then letting him out to do his business but it doesn't usually work. I want to be able to just tell him to "load up" and off we go. No poop, no mess. Help! How can I help my dog to learn that what he's doing is not acceptable?

Jim