I had one slightly like this years ago.
For a start, a pretty much secret engagement is not a full engagement. Part of the point of an engagement is that it's public.
If he's not prepared to tell people you're engaged, or even together, due to it potentially causing issues in his divorce, he should get a divorce ASAP (and I mean ASAP) and then introduce you to everyone.
He isn't considering your feelings at all.
My one claimed that I 'just wanted to get married' or something when I told him how unhappy I was with the lack of true engagement/setting a date.
No, I wanted a relationship that seemed like it was going somewhere, had some progression, and marriage was part of that. It can be seen as the ultimate way of saying that he loves you- to the world.
Your age/wanting children makes it even worse and you're right to be concerned @Ems9Jas . We're supposed to pretend TTC as someone approaches 40 is plain sailing, but your chances do tend to fall fairly quickly and it is something you need to be thinking about.
My one even backed out so I had to cancel the church at one point (my uncle said after I'd done it that I should've got the bloke to do it.) So don't even assume that him getting a divorce is the end of the bullshit.
It is a form of taking you for granted- he thinks he can treat you however he likes and you'll still stick around. So, he doesn't have to divorce/remarry. You've put up with it this long after all, so he thinks he can carry on this way.
He is making a joke of your engagemet, relationship, and you.
Get angry @Ems9Jas and act on your anger.