RandyGal - I don't think your marriage is over like other people seem to think. I also don't think giving him an ultimatum is going to work - and I don't think you really want to leave him just over this.
So, I think you just have to work on him. It's not going to be an instant cure, there's no such thing for this situation I fear.
I would take things very very slowly, start off with your usual missionary position and then try and change position slightly half-way through. Get him used to the idea that sex doesn't just HAVE to be missionary! Slowly work on different positions. Then, eventually, he may open up a bit.
Other than that, RG, not much I can say. Just keep doing what you're doing!
All the best!

CasperTG

__________________
You know, Blackadder, for me socks are like sex. Tons of it about and I never seem to get any.

My Photos:
https://imgur.com/a/80hiRdm