I understand you wanting the security of feeling like he is in it for the long haul. However, may I offer the idea that it often proves to be a false sense of security? People in mono and poly relationships declare their commitment all of the time, and guess what, if a relationship is going to fail, it will fail regardless of commitment.
The fact that he is moving across country to be nearer you speaks volumes. If that isn't commitment, I don't know what is. I mean holy shit, he is upending his entire life to be with you!
If over time, his focus on you becomes distracted, it is going to happen whether he makes promises to you or not. See him for who he is. Allow him to be who is. This behavior is what will keep him in it for the long haul.