From The Collaborative International Dictionary of English v.0.48 [gcide]:
Stepson \Step"son`\, n. [AS. ste['o]psunu.]
A son of one's husband or wife by a former marriage.
[1913 Webster]
From WordNet (r) 3.0 (2006) [wn]:
stepson
n 1: the son your spouse by a former marriage