Gonzo & Fozzie both go nuts over knuckle bones, I couldn't imagine them not! Maybe it's just a shock? Have you ever fed him raw before? Imagine you eating processed foods all of your life (not offending kibble-feeders, as I do too, but it's true.), say McDonald's, and one day being offered a big garden salad? It would take time for both your taste buds and your tummy to adjust. If I were you, I'd put it back in the fridge/freezer and offer it to him in the morning before he has eaten. Maybe smear a little bit of peanut butter on it to entice him to at least try it. =0)

Also, OMGz knuckle bones are the best! They can seriously erase years of tartar in a 15-minute session. Just be sure he's chewing well (if he ends up liking it) and take it away after 10-15 minutes. You can put it in the freezer and save it for another time.