Is Sion support?

More and more I see people playing Sion support and more and more I see it working. I've asked some people if he was support, most people have said no but some people still say yes. When I go look at supports in the store he's not in that section. So was Sion made to be able to support and top like Leona or not, I mean he does have a slow and 2 knockups, and that's more CC than some supports have. Ex. Lux who only has a root and a slow.