Well OP, that doesn't bode well for your future. Are you marrying a grown ass man or a little boy who needs mommy's approval for any important decision?

Either he stands up to her or get rid of him. If he doesn’t, you’ll be regretting it the rest of your life with him

The point of getting married is also to symbolize independence from your parents by forming your own adult life with your partner. It doesn't sound like he's ready to do that, and I'm sorry to tell you but that never works out well in the long run. Before talking engagement, you should really figure out whether you two are on the same page here. His mother might never change her mind, and I get how hard it can be to accept that and move on from it, but at some point we all have to let our parents be upset sometimes and do our thing regardless. I hope you can work this out - if not, tell him to have a happy forever after with his mother. This is one fight you can't win if he's unwilling to take it on himself.

ultimatum time. you or her. personally I think if he's too afraid to afraid to upset her over your engagement then you'll never walk down the aisle.

The real reason is that his wife wouldn’t approve.

What's he going to tell her when you're married? Is he going to hide the kids from her too? You can't build a life with somebody you keep hidden away, you need to let him know you're not a secret. Either he tells his mother or you stop pretending you have a future together.

why are you engaged to a baby man? go find a real man. one who won't deny you.

Here's the thing, O.P., this is a picture of your future if you dont nip this in the bud NOW.

Leave now. No relationship started on lies and secrets will be a happy one.

OP if he puts his mother's feelings ahead of yours now that wont change after the wedding.