His position is eminently reasonable. Marriage is, first and foremost, a financial contract. All those 'protections' people keep talking about, sinply put, means the safety blanket that the lower earner acquires with the opportunity to take assets from the higher earner in the event of the marriage breaking down.
In this case, you're both middle aged with kids of your own. Any 'protection' he offers you by giving you a claim on his assets, comes at the direct cost of his own children. He's probably already been stung with his first marriage - I doubt his pension would survive another raid.
I'm 100% with him - I will never marry again. As a high earner, there's nothing in it for me. And, at 40, I'm too old to believe in fairy tales - I know the odds are not in favour of the happy ever after. So my kids will come first, and not getting married is a perfectly reasonable and rational way of protecting what I've built up for them.
It doesn't mean he doesn't love you. It doesn't mean he wants to grt married, but just not to you, as other PPs have claimed. It means his eyes are open, and he is not willig to put his kids' future at risk for anybody who might subsequently take what he's worked for, to provide for them.
Of course, you are equally as entitled to want marriage, and for that to be a deal breaker for you. If marriage is vital to you, then he probably isn't the right guy.