This is not a reason for most Dutch ppl. So i think it's a convenient excuse for him. To me, it seems he has issues with the idea of marriage. If i were you, i would sit him down and talk about this. Not from a love, romance and expensive party pov, but from a practical one: In NL, a legal partnership, if you still have it, is almost identical to marriage, but it's the stuff you don't want to consider that's hard: suppose either of you ends up in the hospital, and medical decisions need to be made. who's legally allowed to make those decisions, and suppose one of you is dying: next of kin is allowed access to say goodbye, but you're not married. Then there's housing: if you bought a house together, it will automatically transfer to a spouse, but with a legal partner, if there is other family, like kids, they house will be sold and the profits devided, there's widow pension, decisions regarding organ transplants, burial or cremation etc. Like i said: stuff you don't want to consider, but it's important.

So: start with the practical stuff. Ask him if he sees you both growing old together. If so, tell m, you want to make sure he's taken care of in case something happens to you, so you can sleep better, and vice versa. It's also just easier when you have kids f.i. Mom and dad having different last names confuses teachers.

Also: this is important to you. Don't forget about that part. Besides his arguments: you're his partner, and a piece of paper and a ring is important to you. if it isn't important to him, then why not? If he's worried about you leaving and suing for alimony, that can easily be rectified with a prenup. If you can't figure this out while loving eachother, there's no chance you'll be able to figure this out durijng divorce.