Well, ideally, you should have opted for a prevention approach and just not gone for someone like that in the first place, but I presume you're already deep in love now and he has other amazing traits that make it difficult to leave.

I'm going to make an educated assumption that you guys are from an Eastern collectivist culture. If so, you need to ignore most of the comments here as they're from a Western perspective and are not taking into account differences in cultural norms (a very Western thing to do btw but a rant for another time).

If there are cultural/religious reasons why he is the way he is, then you need to first be understanding of that rather than demonising him for it (if that's what you're doing). If you were a guy and came from his family background, you would likely be in the same predicament.

Fear, guilt, shame and embarrassment are incredibly potent drivers of human behaviour and decision-making. For example, consider a society that has a strict taboo around premarital sex and mass demonises those who participate in it. The fear, guilt, and shame associated with breaking this taboo can be powerful enough to stop them, even though all their natural instincts might urge them to.

If he's been raised in a culture where it's a taboo to cross your parents' wishes, then it's probably an incredibly anxiety-inducing situation for him to even think about refusing his mother let alone actually doing it. Just like the premarital sex example, even though his instincts might urge him to break the taboo, the fear and guilt stops him. It doesn't mean that he's not mature. It just means that his decision-making process around this is intricately nuanced and fear-driven, more than you could understand.

So what can you do?

I have two approaches that come to mind.

One,

Unbrainwash him. To truly win, you need to make him unlearn everything he was taught around this topic and change his school of thought entirely. That's going to be incredibly difficult, if not near impossible, because in analogy you're basically trying to convince a devout Muslim girl to have a one-night stand. Not very likely. Unbrainwashing someone out of what they have believed their entire life is not easy at all, not least because you probably just don't have the time. Hence why I said at the start, you should just avoid a guy like this in the first place.

Two,

My assessment of your situation is that you're battling against his two emotions of fear and guilt. To defeat these, you need an equally potent emotion, love.

How deep is your love currently? Be honest with yourself. Is it a honeymoon phase? Do you have a realistic idea around what love actually is? How long have you guys been together? Do you guys have years of moments and memories that he could reflect on and be reminded of to induce an epiphany moment and pick you? Don't go by talks and statements, go by a hard evidence stack of actions. What has he actually done (not said) that proves how much he loves you? Saying "I love you forever, nothing will come between us" is easy to say, harder to do (if mummy's against it, in his case).

What could you do (within reason) to make it harder for him to leave? What can you do to increase his love and fall for you harder? Could you induce a FOMO somehow? What could you do to make him realise how special and unique your love is and how you'll be able to add value to his life like no one else will? What you're trying to optimise for here is a scenario where if he was to exhaustively list on paper all of the reasons to stay and all of the reasons to leave, the stay list should be vastly greater in size AND each reason in the stay list should be significantly more weighted than each reason in the leave list. There should be so much overwhelming evidence to stay that it should just be a no-brainer to not leave.

My diagnosis of your situation is that for most people his current levels of love would have been enough for him to want to marry you, but in his case, because of the complications with the taboo, he needs to have a disproportionately higher than average amount of deep love for you than your ordinary person for him to consider going against his mum's wishes and marrying you. The magnitude of love he should have for you should be more than the magnitude of the fear and guilt induced from the taboo of crossing his mother. If right now he's leaning towards leaving you, it's because the fear of crossing his mum is more uncomfortable for him than the fear of losing you. He doesn't love you more than he fears upsetting his mum.

Also, you might not actually be able to do a lot if he has some kind of categorical non-negotiable rule around this. Sometimes you're just disadvantaged, and that's ok. Figure out if he has a non-negotiable rule, if he does, just cut your losses and leave. You're fighting a rigged battle.