I can't wonder at him being this way after what he's been going through with his ex - and still going through it by the sound of things. He's probably got all his time tied up with his kids and not even able to contemplate the complexities of another 'committed' relationship of the type you want. Probably a bit scared too.
I think all you can do, if you want to keep him (which it seems you certainly do) is just to carry on being his right hand and giving him and his kids lots of support. Try to get as much time as you can with him both with and without the kids around, see if you can't get a weekend away with all of them (ie him and the kids) as though you were a family unit. Little things like that could possibly show him what he's missing.
It's great that you seem to get on with his kids and, by the sound of it, they need you as a stabilising influence in their lives. Try to maximise on that.