Meh, I don't know....it seems like he's doing a "grass is greener" type thing, and yeah, unfortunately it does sound like you're his second choice. It's totally ridiculous how he can be so sure of his decision one week and totally flip flops the next week.
  Personal story cause it reminds me of your scenario: When I met my bf, we went on a couple amazing dates in the first week & began texting/fb chatting every day. So we had been dating for about a week when he started going on dates with another woman that he had met a little before me (but hadn't started dating til after he met me). I actually did not know about her; when we first met, he told me he was seeing a couple people casually, and I was fine with that because I didn't want to rush into a serious relationship too fast. Quickly I realized I wanted to lock him down (lol) but went along with the casual thing... our conversation was to "see how things went." So, open to relationship but seeing how it went first.
  So, I knew he was playing the field kind of, but did not know he was dating this other girl at the same rate and intensity as me. BUT, she knew about me from the beginning. About a month into it, I was sensing hot/cold vibes from him... we got along so well, had a ton in common, amazing chemistry, and when we were together it felt perfect. But he would be unavailable on a Friday or Saturday without explanation & would sometimes go MIA in the evening and then text me the next morning like everything was fine. I was getting weird vibes. Came to find out later, it was because he had been seeing this other girl and was really into her... he couldn't decide between the two of us. Sadly she is more his "type" physically, so he felt more attraction to her, and was REALLY close to choosing her over me, but we had a much better emotional connection than they did. Wellllll, he still couldn't decide about 5 weeks in (yep dating 2 girls same intensity for 5 weeks) and she ended up giving him an ultimatum: either me or her. I probably would've done the same if I knew! Anyway, he understood her reason for the ultimatum, and decided to choose me since we had connected way deeper. He told her he just wanted to be friends, but also that it was "dangerous" to hang out due to the physical attraction. They became "friends", she tried to hang out and flirt with him over the next couple months and he shot her down each time.   So in my case, yes he "chose" me and did not display any regrets for doing so. He definitely didn't contact her in a romantic way after that, and shot down her attempts. I only found out about her when we had the exclusivity talk, but did not have any idea of the extent (or that he could not decide) until MONTHS later when unfortunately I snooped and saw messages between him & a friend. He's not the type of guy who usually dates multiple women at once but had been torn between us to the extent where he was analyzing it with this friend.   He chose me and we do have a great relationship, but I still feel insecurity to this day about what went down in the first month. He still looks at her Facebook photos regularly and part of me will always wonder if he wishes he chose her (he will never ever admit to that). We're in therapy, and I realize a lot of my doubts come from my own self esteem issues, but it's never a good feeling knowing your man couldn't decide between you and someone else. It will always mar my memories of our first few weeks together.
  TL;DR... it's in your best interest not to give him a chance. You may always wonder if he wishes he chose her instead, and as someone who was chosen up front, the doubt still lingers somewhere deep down.