He's very much not like Superman. There are many other DC superheroes Tony Stark can be compared to in terms of abilities, like one of the Green Lanterns or Steel. There are too many characters in the Marvel universe that are actually Superman-like in terms of superpowers but not as iconic in personality or presentation. The closest Superman counterpart for the Marvel universe I can think of right now would be The Sentry. Imagine the blonde bodybuilder Christopher Reeves fought in Superman IV (that Lex Luthor created using DNA from Superman's hair, combined with a nuke thrown into the sun creating Nuclear Man), only a good guy.