The reason why I ask is that I live with my partner who is the step dad (to be) for my two, who live with us.
This once happened to us. We'd been away on our first holiday and the night we returned, he announced he wanted to leave. He said he was struggling with his relationship with my two as well as being pressurized (into sex for example) by me.
I was naturally upset but had the balls to say if he wanted to go, he could do but I would not want to see him again because he would mess my childrens heads up as well as mine. To give you some time scale, we'd met in May 1999, started living together in March 2000 and the thread was April 2001.
He stayed and we're still together today.
I'm a stepmum to dp's son and I find it hard at times although the little lad's only 6.
My personal opinion is stick in there. These relationships are hard. Make sure you get QT with dh and you be 'adult' together (not just physical but intellectual i.e. communicate!) My relationship with ex. broke down just after the kids came along - maybe he was feeling pushed out?
I think he wants to be with you - he's still there and you've both got something to work on. He sounds a good guy between the lines. He's probably finding it difficult to cope with the terrible twos, and that makes him 'normal' as they are trying times for everyone.
As long as you make time for each other, you'll probably have a great life together. The best of luck - step relationships are difficult.