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?

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.