*UPDATE BELOW* Trying to make the best decision while not sounding like a horrible person

I'm 24 and I've been dating this great guy who is 27 for almost two years now (roughly 20 months). He does have a son with his ex.

Things have been amazing between the two of us. We are best friends, we're hard workers, we get along so well yet have just enough differences to keep us interesting lol.

Well. For the past 2-3 months there has been a lot of stress from his ex girlfriend (who yes is the mother of his son). I'm not a jealous person. I respect her and what they used to have and the fact that they share 50/50 custody of their son and are making everything work (without any court intervention). I think it's awesome.

But his ex has been doing some not so great things recently. Like drinking HEAVILY out of nowhere. Having a very toxic attitude towards everyone (friends, family, etc.). She quit her job without warning. She's tough to get ahold of (when my ex is supposed to pick up or drop off their son) Now there are people pushing for therapy for her and AA and all of this drama.

Their little guy is amazing and such a sweetie. He has grown on me. I wouldn't ever try and be the woman who replaces his mom. That's not me. Right now I'm still daddy's girlfriend and he just calls me by my first name which I think is perfectly fine. But now with all of these issues with his mom, I sense that he is scared and confused and very vulnerable. Because more recently he's extra lovey towards me (mind you he's always been a hugging kind of kid). He just seems softer towards me. He wants me to be with him all the time and wants to tell me every detail in his little world. He's a sweetheart. But for the past week he's been going on about how he wishes he could pick a mommy. And wishes he had a mommy who didn't yell (he sadly witnessed his mom drunk and screaming recently and he's been shaken since then). He asked me directly if he could call me mommy.

I was honoured that he sees me as someone he can trust and turn to. But now I'm terrified that he thinks I'm the replacement. I don't want him wanting me to be his new mommy because I know that would kill his mom. I don't want her using her son to drive a wedge between me and her ex who is now my bf. I'm highly considering leaving. Because I don't want to hurt him by telling him I can't be his mommy. I'm afraid I can't be there for him the way he wants me to be.

*updated portion*

Thank you first of all for all of your comments, thoughts, opinions. I appreciate them. I woke up this morning, my boyfriend and I made breakfast together like we do every Sunday. And I laid it all out on the table very honestly. I told him I was honoured but I was scared. And I told him how I felt on the inside and how I wanted to include him in my thought process. I also said I was thinking about ending us in order to not overstep. He was glad I was honest and talked it out. He told me that he loves me and there's no way I would ever be overstepping and that he couldn't have his future wife and the future mother of his future kids leave. He apologized for stressing me out (even though it's not his fault). We're going to take one of your advice and talk with his son about calling me "Mama (name)". I'm not a replacement. I'm a bonus because he has a mommy and a Mama (name) who loves him dearly ❤❤❤