I think responsibility is very important, and should be expected of children. I also believe that children should always be respected, even if they make you angry. My opinion is that you very respectfully explain that it is unfortunate that he did not take responsibility for making sure he had his hunting gear, and that subsequently, he’ll have to sit this one out. I’d also explain that you were really looking forward to him accompanying you, and suggest that next time you’ll try to remind him.
Also, it doesn’t sound like he really wanted to go, or at least not bad enough to remember his stuff, which to me, is another reason to sit this one out. You jumping through a bunch of hoops to get him out and hunting, when he really doesn’t seem to want to be there sounds like a recipe for spoiling him, and provides no reason on his part to change his actions.

He’s your nephew, so you’ll know what is best, but that is just my opinion.
KP