Sounds like a fear of heights. Seems to me if he didn't want to kill one, he wouldn't be there to see you do it either.

Have you asked him why he didn't want to hunt out of the stand? Won't he open up to you when he realizes there's no harm in ANY reason why he wouldn't do it, and that you just want to know so you can come up with a solution (e.g. making a ground blind if his fear is heights)?