If it was a consumible like a soul gem or a potion, they will use them as needed. Super bummer when my dad figured that one out. He gave Lydia a bunch of potions to hold on to so she could be like his medicine women, but she svcks and gets the crap beat out of her all the time. He was in a battle, went to get them back, she had already used them all. Bummber.
If it was not a consumilbe, but something that is worn, I have had this happen. Gave my companion a super sweet helmet to use, we get into the first battle with Alduin, it's over, she walks up, no helmet. I think it was knocked off of her by a shout or something, but I don't know of any shouts that do anything but disarm.
If he doesn't have it on him at the moment, I don't think taking him out to die will allow it to appear. It's worth a shot though. At the very least it will be satisfying and due punimshemt for losing your stuff.