Welcome to the club! lol. I am a step mom too and I love it. Don't sweat it to much, just take it as it comes, you don't have to go out of your way to make him like you/love you, just be there for him, treat him how you would treat your kids if you had them. Even becoming a new mom is hard, when you have your 1st natural child, teh questions are not all that different. Technically you are a new mom, so congratulations! Never view yourself as "just" the stepmom, and never see the natural mom as the "real" mom, becasue what is "real" ? He can hear you, see you love you, hug you,touch you, so therefor you are "real", you are mommy #2, or a bounus mom, or "insert name here" Whatever he feels comfortable calling you(so long as it's nice :)) Don't worry, it does take getting use to, and you may hit a few snags or feel left out now and then, but like everything else it takes time. I have been a step mom for 2 years, and it took about a year for my step child and I to have a great relationship and understanding. He calls me Lysa, and tells people I am his other mom, and actually, he come to ME, not his dad or natural mom when he has difficult or deep questions. It's just a matter of how much love you give them, how open you are with them and *(REALLY IMPORTANT) how you treat the other mom. If you alwasy are kind and respect her, the child will respect you!
oh yeah and YES your opinion counts in his life, don't be afraid to speak up on his behalf! You do have a say, cause you ARE an mom!
Good luck and congrats on your future marrige and family!