I have no idea what to think about that! He sounds very transparent, which is good. I am eager to hear what everyone has to say....

He may say they were never serious, but there is clearly still something between them. If he seems worth the effort, I would establish clear boundaries now and see what kind of push back you get. When they go on trips and when he stays over, do they have sex? Has he had other relationships since his kid was born? If so, why did they end? I'm trying to figure out how long this has been going on and if it was a problem for other relationships.

If I had managed to pull myself out of the misery that is a blended stephell, I would have radically different standards the next go round. To that, I wouldn't be interested in a man who had such a loosey goosey arrangement with ANY other partner/ex/barnacle buddy/fussy MIL. (I had a bunch of other red flags typed, but your list is different than mine.) You do not deserve a life when you have to figure out how to enjoy the sand and smell of the cat litter sammie this guy is so earnestly offering you. His kid may be well behaved, his not-ex might be staying in her lane. But! He hasn't taken much initiative in his own internal world to MAKE room for a number one lady. I'm glad you are feeling warm and tender from being treated better than before. I offer caution about this one. Make sure you aren't comprmosing yourself. You are more fabulous than you are giving yourself credit for. He could be falling all over himself to "woo", and yet you are being fluffed to accept this not-ex in all the holidays and outings that ought to be yours and his. And this is early days still. Your "honeymoon period" is mighty flimsy,weak, and short.

Some couples are close to the EX  Have holidays together. Babysit for each others.   I for one could not do this.  I feel [maybe wrong] most people on this board would be happy if the ex disappeared. ... I could never have dinner with the ex. .. Once divorce the friend ship ends.   Once you have a sexual relationship,, ...you can NOT go back to friends.  

'My wife in the beginning took the kids to see their father and she stayed there.  I told her I did not like that.  She did it again. I said it's either him or me. Your choice.  
I do understand there's events that you must be together.   Graduation, wedding, birth of GK   But that a short list.  You may attend sports events. But you don't have to talk .. actually today. Onc th kids are older. Texting. And saving all the texts is the way to go nnyou have a record of what's agreed to.  

He's honest about their enmeshment, which is good. He's invited you to be a part of it, which is i guess good. Maybe you will get there and see that they are just 2 friends raising their child together and you will be welcomed into the fold.

 But what if BM doesn't like you? What if she sees you as a threat and tries to "run you off?" What happened to me was that BM would be rude to me as soon as SO turned his head and then sweet as pie when he returned. I wish i had run. He finally set boundaries but i don't like the person i had to become to get it done, and i'm still angry i had to.

Also, even in that best case scenario of the 3 of you raising this kid as besties. Could you stomach it? 

I'd run ... far and fast. She's already caused problems in his marriage  (why did he allow that?). Why would you ws t to play second fiddle when you could be the soloist? I really don’t think it's worth continuing. You deserve so much better.

"He also said I am invited to any activity or trip." 

Are you kidding me?!! Does he really think that his new girlfriend would like to go spend the weekend at his "friend's" house? You know, the friend he has a CHILD with after having a sexual relationship with her? 

That statement in and of itself is enough to make me run, and it should you, too.  This guy thinks he can maintain whatever relationship HE WANTS with this woman and not only should you accept it, but JOIN IN with it too? 

Do those activities include all of you sleeping together? Pajama parties, so to speak?

C'mon now. This whole situation reeks. 

This is a big NOPE for me. I don't like the 'she's like family to me' statement. What are you then? Is she going to be ranked higher to you as he considers her a blood relative- even though he at one stage saw her attractive enough to have sex and conceive a child with? That's so conflicting for me and would make me feel a bit ill. 
Also, they have trips away together with the child they conceived.. or without the child? Does that really sit ok with you? Are you his partner or her? He must love her in some way to consider her as family. It's weird and you will eventually wind yourself crazy with speculation and always wonder where you stand vs her. 

 Hon, the fact that you are even considering this ridiculous, sister-wife setup has me wondering about your mental health. You’re commending your boyfriend for his honesty in confessing the attachment that he has with the mother of his child, well whoop dido! 

How about if he made a clean breast of enjoying a sado-masochistic relationship with the woman, complete with whips and black leather and invited you to join in their dirty weekends at Motel 6? Just because he’s being honest about a completely unacceptable liaison with his ex does not make it right!

Have you discussed this insanity with your mom or a close friend? If I were your mother, I’d have you Baker-acted. Sorry to burst your bubble but this mess is even worse than your previous romantic association, where you felt ‘preyed upon, duped, and naive.’ 

Hon, there are plenty of fish in the sea; get back out there and toss your hook into the water again; this one’s a throw-back. First, however, a few sessions with a therapist would do you the world of good in learning how to respect yourself enough to find a man who is not already spoken for.

Run!!!

Time for a T column analysis and comparison. 

List on one side the key characteristics of your past relationship and toxic StepFamily experience. On the other, list the characteristics in play in your current relationship.

My firm position is that if you are going to remain in this replay situation, your current SO needs to have clarity that you and the relationship trump his failed family.  In all things, all of the time.

Enmeshment with his X is a fatal flaw in any relationship he may engage in.  Currently that relationship is ....  you.

The odds of a high quality fully committed relationship with someone who is highly enmeshed with their X are slim and none at best.

IMHO of course.

Learn from your past, do not repeat that failed journey.

Take care of you. Your current BF is focused on servicing his failed family. That, is a recipe for disaster.

Good luck.

How old are you? I ask because I don't think you realise what a precious commodity you are  - a woman with no children. There are men put there who are climbing over each other to find a woman like you. Please be patient. Don't sell yourself short with a "better than nothing" solution. You deserve better than someone who's getting all his relationship needs met and who expects you to be content with sex and being second best.

This guy is a HOBOSEXUAL of phenomenal development.  Likely needs a roof over his head, a financial sugar mama, and he is getting laid. Probably by both his XW and the OP.

Run Boostgal. Run!

You are a HOT commodity in the new relationship world.  Don't settle. Thrive!!!!

He admits this dynamic was a problem in his prior relationship/marriage.  I think that's a pretty big red flag.  

I would question why two people that made a child and seem to get along so swimingly don't make more of an attempt to be together and raise their child.  Surely at one point the relationship turned romantic... at what point would he be better off nurturing that into more of a loving relationship vs dragging numerous partners through the mud puddle of his prior relationship.

I mean.. I get that it's good when the bio parents aren't toxic at each other's throats but I think this goes too far in the time they are spending in close quarters.

In the end, do you want to be in a relationship with "one man".. or do you want to be in a relationship that will always include another woman.. that will likely have a say in how your home is run.. where you vacation.. when.. etc.. I would not be happy in that kind of situation.. and I think it's reason enough to want to cut this off.

YOU sound like a catch. Him...not so much. 

Here I am about 3 years later, and I am dating a man w/ a young son. There seems to be some enmeshment with the BM. They were never married or in a serious relationship. However, he has described her to me as "like family." They sometimes go on trips together and he will stay the weekend at her house for his visitation at times. I have expressed this crosses boundaries for me. 
 

Please--if you've been here in ST before because of step issues, rethink this. "Enmeshment" is a very very bright red flashing light. "Like family" concerns me as well. These are words I would have used early on with DH and his situation, and the BM is/was a raving toxic lunatic who made our lives hell for a few years. She definitely felt like she was being replaced when I came along, and I'd bet that this BM would feel the same way about you. BM here, too, wanted DH to stay at her house for skid visits. Her desire was to "play family." Well, guess what? They are not a family. Or maybe they are. Which means where do you fit?  Trips together? This doesn't make sense. This sounds like dysfunction all the way around, and you haven't even MET BM yet. 

I know it's easy for me to say, but I'd suggest you find a man who does not have kids.