Hi L.,
I think it's wonderful that you are concerned about your role enough to ask for advice! Your situation sounds very similar to mine. I have had many of the same concerns and worries that you express. I married a man who had a 2 1/2 year old son at the time. My stepson's mother has always been over the top and continues to be. And, we have just welcomed our daughter into the world. It has not been easy at all. We've all had to make many major adjustments to make this work, but I think it's worth it.
You will only ever have as much influence on your stepchild's raising as your husband will support. Too many stepmom friends have their husbands undermining their rules or contradicting them or not enforcing what they have agreed on. Some of my stepmom friends choose to continue to connect with their stepkids. Others prefer to detach - sadly. So, remembering that, proceed slowly and see how responsive your boyfriend is to your requests, concerns and input. If he welcomes you as a real partner in raising his son, take that on, but remember, his say should be the final one because he is the boy's father. Does your boyfriend say to his son and you that the three of you are a family? Or, are you the outsider, come lately? Does he include you in activities while also reserving special time for him and his son?
Remember, he still has that connection to the boy's mother and always will. He will have to make decisions with her and communicate with her, etc. My advice on that is to stay out of that situation. Wait for your boyfriend to ask your advice, and never respond directly to a crazy woman. It won't get you anywhere and sometimes it will even backfire with your boyfriend taking his son's mother's side! When you get really angry with her, try to stir up some compassion for her. Think about how her ups and downs must affect her son. Giving him a calm, consistent, positive influence will go a long way in establishing a relationship with him.
Think about what kind of life and family you want to build. Is your boyfriend's perspective similar? In order to have what you are looking for, it is crucial that your boyfriend be on the same page. If not, there's nothing you can do to make it happen. So, before you "step" in, test the waters and evaluate.
With respect to his son, just love him. Give him lots of hugs and encouragement. Play with him. Read to him. Snuggle with him. Leave as much of the discipline to his father as possible. Even though you are in his life at a young age, you will not have the same kind of bond he has with his natural parents. Love and enjoy your stepson. Give him the best example of what a good marriage is and what a good woman does.
Find some things you can share with him - just the two of you - hobbies, activities, inside jokes, etc. Bring some traditions into the family. This has helped me bond with my stepson. I looked at what he liked to do and built some family traditions on that. For example, we have family movie night - every Friday he's with us. He gets to choose the movie and we all eat a special treat -- On the Couch!!
Find ways to inspire your stepson. Find ways to foster his relationship with his dad. I have always let them spend their time together: on the first day each time he comes to our house for his long weekends, on the last day before he leaves, every morning (they watch cartoons together on the couch and snuggle), and every night my husband does the bedtime tuck in.
Include him, include him, include him. If you get married, find a way to include him in the ceremony and preparations. We still refer to it as "our wedding" as does my stepson. My sister's son was four when she married his now adoptive dad. Before the wedding my nephew used to tell everyone, "Me and mommy are marrying Bill." And, that's exactly how it is. If you have children, include him in that. We referred to my daughter as "our baby" and so did my stepson.
Now we have a new baby together. And, my stepson is her biggest fan! He told us the best thing that happened to him last year was that he got a baby sister. He tells us she's his best friend. (It helps that his mother supports his relationship with his sister even if she doesn't support his relationship with me.) Our daughter, of course, just worships her brother. And, my husband loves them both more than he can put into words.
I can also recommend a good book. The single girl's guide to marrying a man, his kids and his ex-wife by Sally Bjornsen. Great read!!