Heidi, your comment that, "Another woman's child calls me mom." doesn't go far enough.  You aren't just called mom.  You are his mom.

Being a parent is at most tangentially about biology.  Conception takes only a moment, after all.  Parenting takes a lifetime.  In the end, it is the lifetime relationship of parent and child that really matters.  That relationship overwhelms any biological relationship.

For me, what it means to be an adoptive parent was best expressed by the author and journalist Jim Bishop.  "I have four children, two of them adopted. I forget which two."