I think the fact he has just basically moved into yours and has his laying empty as his little bolt hole speaks volumes.
If he really wanted to move this forward and really wanted to be living with you he would be suggesting selling his and looking at getting somewhere together. As partners. As its tands, he has his little safety net and I bet the minute a conversation gets difficult or he has a bad day he'll just bolt off to his house. he is commiting to nothing.
At the moment he is getting the best of both worlds. No commitment from him, but the comfy home life at yours.
And the fact it hasn't occured to him that you are footing all the additional cost of him staying at yours, when you are on half his salary AND supporting children is really very shit. He is either really really thick or knows exactly what he is doing and doesn't care.
What would happen if you suggested he sell his house and you look to move in together properly? Whether you want to or not, try it and I bet he'll say hes not ready and even panic!
I'd just have a frank conversation with him. Hard as it is. As offended as he may he with you wanting your space. Explain, he has 2 houses. he can have space whenever he wants. you can't. Either you live together, equally, or you don't. Hes getting all the cake at the moment, wheres your slice?
I had a similar scenario, basically staying at mine almost every night. he'd come in with the odd takeaway or take us out for sunday lunch. Great, I felt I should be grateful and felt guilty for it pissing me off. but ultimately his takeaways and meals out were not contributing to my bills or his stays, which I was struggling with, so although nice, it wasn't actually helping me one jot financially iyswim.
Don't tell him about the inheritance, either.