What would cause yellow/orange skin in an 9 month old?

Carotenemia. Carotenemia is usually a benign condition characterized by yellow pigmentation of the skin due to increased beta-carotene levels in the blood. This usually follows excessive consumption of carotene-rich foods, like carrots, squash, and sweet potatoes. By eliminating these from the diet the color goes back to normal. If not see your doctor.
If his palms are... "yellower" than the rest of him, and eye sclerae are white, this is carotinemia - build-up of carotene from vegetables, which is harmless. If eyes are yellow also, see your dr right away.