Would adoption leave apply for adopting my bride's son?

When my mother and step-father got married, my step-father signed a bunch of paperwork to legally adopt me.

I now find that I may eventually be in the same situation. My girlfriend has a 10 year old son. If she and I were to get married, and I legally adopted him, do you think my company’s adoption leave benefit would apply?

(Obviously, I know only someone in my HR department can answer this question for sure, and I’ll try calling them Monday morning. I’m just wondering about other people’s experiences and guesses.)