We don't know for sure. Could be CHIM. Or perhaps Akatosh ascended him because he was such a hero to humans who didn't really have their own chief Deity. If you really want to think about it. Because he was Dragonborn he was the reincarnation of a dragon, one of Akatosh's children. So he was already kind of a demigod. To make the leap from that to fullblown god isn't that difficult. --- I made me do it..... wait.