I want to pop in (sorry if this is long!) I don't mean to offend you at all, and if I do, please disregard what I say then!
My fiance (we got engaged in January, 3 years into dating) and I were rocky too until I became pregnant. We dated for a while, he half-cheated (was receiving naughty pictures from a friend) and that started the lack of trust. After a few months, we broke up and were on and off for a year. We couldn't fully break it off even though we went on dates with other people. We started to see each other again, my lease was soon up so we decided to try moving in together. I know it sounds crazy but it worked! Saving money on both our parts by sharing finances and bills really helped. A few weeks later I became pregnant. Our relationship is now stronger than it was before, now that Elliott is here. Yeah, we still go through money struggles and we still fight from time to time but he told me that he only has good things to say about me to his friends. Of course, I still get a wild hair once in a while or get afraid something will happen. That's when it blew up the first time, when we were perfect and I found those pictures. I am used to things going to hell when things get good. But we are much more stable now, even though I have issues to work on (weight loss).
Long story short, I think you may need to go to counseling. Figure out if it is PPD or if it is just your head. Trust me, I went to counseling for a year two years ago due to stuff in my past that I just couldn't shake. It was affecting my relationship as well and I got my head screwed on straight. I don't want to say you guys have a bad relationship because I can remember when I was not very trusting and I questioned everything that my df did. I even asked why he was home late one day (he works overtime randomly) and asked if he went to the bar, since he used to love going to have a beer and listening to live music and hadn't been in 9 months or so. It's his responsibility to pick up Elliott from daycare. He said, "What, you think I left my kid in the car and went in to the bar?!" And I said "No, I wasn't sure if you went between work and Elliott..." then I felt dumb. Looking back, yeah, we had a lot of fights, but most of them could have been dismissed if I wasn't so mistrusting. Of course we had legit fights too though.
I don't mean to say that you are the bad one and causing a bad relationship. But I am thinking maybe you will feel more comfortable if you talked with a therapist (I went to a psychologist, he was AWESOME) and my self confidence rose. Once that rose, I stopped picking so many fights and trusted my bf more. That change caused us to be happier even in the low times. He soon learned how to dismiss a "nit pick" fight, were I may be feeling grumpy and use some snarky response (it happens from time to time lol) and he will make a joke then which tosses my grumpiness out the window.
So maybe if you talk to someone and become more self confidant in your feelings and what you want to do, you will be able to see the whole picture clearer-do you really feel that there is a threat of him taking off with Mason? Or are you feeling afraid of being left in the relationship? Are you pushing him to see what his response would be, if he would take off with him or if he would work with you on it? I think once you feel better about yourself, you can see the issues in the relationship better and will feel far more comfortable in what your true feelings are and will feel confidant in standing up for what you want (legal custody documents etc). It may also help clear your mind and see if there really are issues in the relationship. Maybe you are like me and get afraid when things start going good, so you imagine what could go wrong. You feel stressed and you guys are arguing, maybe it could turn into a huge fight and that could happen or maybe that would be stretching it. If Tim brought that up to me as a legit concern right here and now I would feel very hurt. I couldn't imagine taking Elliott away from his daddy, and I would feel offended that he thought our relationship was bad enough to where he felt that it was an actual threat and he wanted to take legal action to prevent it. Think about it that way, what if your bf think things are going really good, despite the fighting (all couples do) and you just threw this huge curve ball at him?
I know the first two months postpartum were really tough for us, and Tim didn't even want kids before I got pregnant. (O.o) But now he says he loves coming home to our family. So maybe once you talk to someone you can bring your bf to talk as well, that way you are in a healthier place to work on attaining a healthy relationship. It would be horrible to go to relationship counseling just to leave feeling like all the issues of the relationship are on you.
OH MY that sounds horrible. I don't mean it to sound that way. I am just thinking of you feeling bad if the therapist pointed out your trust issues. I am very sensitive and once someone pointed out a flaw of mine I would immediately take all responsibility for all the problems!
I hope that helped in some way. If I made you feel worse, I apologize! I just want you to know that there are other unmarried couples on here that have gone through recent hard times. There is a light at the end of the tunnel, although I couldn't tell you where the tunnel leads.
Let me know if you want me to delete mine as well! I wrote this while Michelle and you posted again so I didn't see that. I would have pm'ed it if so.