I didn't go from "open" to "committed" in a poly relationship, but I did go from having casual sex with men to not having it at all, and "settling down" with my husband.
However, that's casual sex. I'm still poly, and still able to fall in love. And well, I hear people have different levels of "polysaturation", maybe his is two. That would mean that just like mono people have just one person, and new people would be considered, when he has the two of you in a committed way new people won't be considered.
However, I'm not sure how easy or possible it is to know your level of poly saturation. I'm in love with three people right now, and when I fell for the third one I freaked out, because I thought, two is one thing, but three? That's "too much!" (Don't ask me why I thought that. Baby steps I guess?)
I'm not saying he's not being sincere here. I'm just leery of "I won't fall in love" and things like that. Because I always think, how do you know? You can't control these things. You can only decide how you act. And setting you up for not being able to be with someone you love (a potential third) seems sad to me.
Would you feel terrible if he had another committed partner? And did he say he wouldn't? Maybe he just doesn't want to be casual again, and in that case I would trust him more easily because it's about actions. And maybe he got what he wanted out of going out, etc, and now doesn't have a need for any of that anymore.
If you guys ended up together in a committed relationship, and that a few years down the road he met someone, told you about her, and asked how you felt about it... Do you think you might be okay with it, or does it sound to you like it would be too painful?
I'm not saying that will happen, simply I'm wondering if your problem is only with going out and meeting lots of people and having casual encounters.