I'm not convinced not wanting to marry (you or anyone else) has to mean he is not 100% committed. DP and I feel that getting married would not increase our commitment to each other at all. We may yet get married some day (have been together 10 years, have 11m DS) for reasons of legal stuff and as an opportunity to have a party, but we may just as well not.
Does he really know how you feel about marrying? Does he know how very sad you are about him not wanting to marry you, does he have any idea that you are even considering that it might, perhaps, be an issue over which you would leave him?
If he knows how you feel, and still chooses not to marry you AND not to explain why, then he is being inconsiderate. If marriage means little to him, and he knows it means lots to you, he should want to marry you for your sake. If he really does strongly not want to marry for some reason, he should be explaining it to you. Even if he has strong feelings against marriage (that might have nothing to do with not being 100% committed to you and your relationship), he is currently simply putting his feelings (against marriage) above yours (for marriage) without any discussion. That is inconsiderate. It needs to be put out there and discussed. You need to let him know that in your book, he is being inconsiderate and he is hurting your feelings, badly.
I can imagine a situation (no idea if anything to it): Say he feels strongly for whatever reason that he does not want to marry you. He knows you (more or less secretly) do want to marry. He knows/thinks that if he came straight out and said that he would never marry, explaining his reason, you would be upset and would perhaps leave him. He is scared about this because he really, really wants to be with you... so he keeps putting it off, hoping you'll come round to the idea that marriage is not so important. Not realising that this is making you more and more upset and that actually the lack of open discussion is threatening to ruin your relationship.
Is not getting married a dealbreaker for you? If so, you need to let him know. Perhaps marriage is a dealbreaker for him - you should find out, confront him about it directly. If he told you he really, really wants to be with you forever, but if you insist on getting married, that is the one thing that would force the relationship to end - would you still want to get married i.e. would that mean that you'd want to leave him?
In short, I think the cards need to be put on the table. Yours and his. Then you can both consider the next steps and how much the relationship means to you. Just going on as is seems to be slowly eroding the fundaments of your relationship, which is a pity and not necessary.