I don't think there's any legal ban on this but it varies from state to state so check with the state where you want to get married.
You are not blood related so there's nothing wrong there.
However, it really depends on your relationship with the son. If your mother married your step-dad fairly late in life and both of you were grown up already, or you were raised far apart and never really spent much time with eachother, I don't see anything wrong per se with this. But if you were raised in a close family together and more or less grew up together, that's getting a little taboo and almost like marrying your own cousin or even closer.