And, another point: Eventually, your relationship may become solid in spite of the circumstances of the beginning. However, this does NOT give you blanket license to call your stepchild your "son," especially if he's quietly harboring some last resentment over your role in his parents' divorce--some people upthread indicated they were happy their stepparents called them their children, but that should be the stepchild's (as well as your) call.
Finally: If your stepchild's mother dies, all of the goodwill you built might go out of the window for awhile. This does not matter how old your stepchild is. Regardless: Do NOT call yourself mom and your stepchild your child during the grieving process unless specifically asked (the possible exception: if you've done it for years and your stepchild would welcome that consistency). And whatever you do, do NOT badmouth the late mother, her parenting, or really anything concerning her, to your stepchild. If you must, call a counselor or a girlfriend or post here...but as far as your stepchild is concerned, his or her mother was perfect.
My mom had been dead for 3 years before my stepmom started making a point of introducing me as her daughter. Also, it was after I had written her a really nice, long mother's day card, thanking her for how she'd stepped in and really filled the void my mother left. (There was not a dry eye in the room after that card!) And she still doesn't call herself my mom, just calls me her daughter.
I H.A.T.E.D. it when, as an older child and teen, my stepfather referred to me as his daughter. I don't know how I would have felt about it, if I hadn't hated him. Because, although I hated it, it also contributed to my feeling of security in my place in that side of the family, since I didn't live there most of the time. But there are other ways to make a child feel secure than to call your stepchild your child when they hate it.