He's not the Richards' son. He their distant descendant from the 30th Century. As Kang, he's an Avengers villain. Most of his schemes revolved around fighting them. As Rama Tut he fought the Fantastic Four. Though that was before Kang was introduced. Making Rama Tut and Kang ( as well as Immortus and the Scarlet Centurion) the same person is a retcon.
I'm not sure how they arrive at what characters belong to what license, but I believe Kang is lumped in with Fantastic Four because of those later revelations that he was Rama Tut and his ties Reed Richards. I guess it's under the logic that Ego the Living Planet was somehow under the FF license even though Ego first appeared in Thor:
http://io9.gizmodo.com/marvel-and-fox-worked-together-to-swap-characters-for-d-1788950819
Kang would be a good after credits reveal on the FF time travel idea, though. It could be under the idea that Kang was manipulating things in a grand plot that ends up involving the Avengers and everyone in the MCU (basically making Kang the Thanos figure of a new MCU connective plot).