Advice needed for moving on from baby daddy to new man...?

Ok, so my son's father and I were together for about a year and a half before I ended things back in Oct. 2010. We were bitter at each other for about 3 weeks after, then we slept together. He's stayed the night on and off throughout the week and on weekends and we've been sleeping together ever since then up till about a month and a half ago. I could feel myself growing apart from him, and within the last 2 weeks, I've felt like he's been distancing himself. I want to keep things cordial between us since we have a child together, especially because we have no custody agreement. I just met a wonderful man a couple of weeks ago, and have been on a handful of amazing dates. I wanted to be honest with "baby daddy" about me starting to date, especially since him and I had been intimate for so long after we technically broke up. I told him asked him what he thought, and he said it was fine, that he had actually been chatting with someone also. I was actually slightly excited because I figured that meant we could both have closure and move on, but now he's constantly on his phone texting (I'm assuming to her) when he's supposed to be watching our son, and he's always walking around with his ipod blasting country love songs which, I find odd because whenever I had country on in the car, he hated it. Also, he's blowing up his facebook page with the same country love song videos from youtube, and I almost feel like he's trying to rub it in my face. He also coincidentally started working out again and writing all about it on facebook. The thing that bugs me is when I asked him questions about her, he gets defensive and simply said it's none of my business. That may be true, but I feel in order to stay cordial and be completely honest, we shouldn't have to hide anything. So that fact that he won't tell me anything is a big red flag to me, especially if things work out between these two, she may be around my son. I'm not ok with that if i don't know her, or what kind of person she is. The other part that really hurts is that whenever I see any and all of this behavior, I get the biggest twisted knot in the pit of my stomach, not because I still have feelings, but because I constantly think that he never acted this way when we dated (maybe i wasn't good enough), even when we first started dating during the so called "honeymoon phase". My question is, what can I do to not let all of this affect me? How do I handle this complicated situation? Any and all advice is welcome, and wanted!! Thanks!