Stressed out!

I just need to vent a little. My boyfriend has a 6 year old son and we just now got him 5 days a week. It's been hard because his mom is a low life who would rather do drugs then take her child to school or spend time with him. She gets to choose when to be a mom to him and it's sad because he does love her very much. I'm a firm believer in the fact that every child has one mom and one dad and I love that he still loves her. Although, it's frustrating because I'm the one whose 7 months pregnant and getting up in the morning and helping take him to school when my boyfriend works and watching him and taking care of him. It feels like I've had to really step up in this role and take on a child because the mom won't grow up. His son and I get along really well and he looooves to hangout and tell me things. Which is great. But sometimes Its uncomfortable because he will say things his mom says bad about his dad.
I've never wanted to step in and be called mom by him and last night he was just talking to me like normal and said "you know I can only have one mom"
And it really caught me off guard because I Would NEVER even want him to think that I'm trying to be his mom. Idk if it's his moms insecurities coming through him or what. 
I'm going to be his little brothers mom and dads girlfriend and wife one day and that will make us a family. But I'm not here to take anyone's spot. No matter that the case is.
Sorry this is so long and maybe confusing but I'm just in a weird spot right now.
Any advice?