I have been reading Steptalk for some time now, but I have not been posting.

But I thought I could ask for advice, so I registered.

So here's the deal. My boyfriend and I are together for more than a year now.
He told his daughters - 14, and 17 half a year ago.
Since then, their relationship has been horrible, and our was getting worse and worse. When he first told them, they were in shock and thought that I'm just a fun game that will last a few days, but since they saw it was serious (I even moved in),they started working to separate us.
At first it worked - his behaviour changed, he was different and he would allow them do disrespect me, threaten me, call me names, and we almost broke up.
But then he took care of it - by setting boundaries and talking to them about it in a firm way. It did work, they stopped doing it,but they also stopped coming over.
For a few months they'd come and just sit and pout, whatever he'd say, they would answer with one word and look away. They wouldn't even look at him! Whatever he offered (food, drinks, to go out) they'd say no. They ignored him and me,just sitting on the couch not doing or saying anything. He tried to talk to them but they refused.
But then, a few months later, they stopped coming over completely. They said that if he has me, he won't have them. They said he had to choose, he wouldn't choose.
He has been miserable ever since, because they are not talking to him and he hasn't seen them. He called them, but they would not talk to him. Here's one of their conversations.
BF and his daughter 17 years old(i'll label her SD , because it's easier) :
BF: Hi
SD: *MUMBLEMUMBLE*
BF: How are you?
SD: fine
BF: How's school, anything new?
SD: Fine, no
BF: I'd like to ask you if you'd like to meet me, come over, or something,and talk..
SD: Did you break up with her?
BF: No
SD: Are you going to?
BF: No,and I'm asking to see you and your..
SD: Same answer to you then

She hung up.
He called again, but nothing. He called the 14yr old, nothing.
He texted them a few times, 14yr old replied "call us when you leave her"
He did not answer to that. He called them yesterday, no answer.

I don't think it's coming from their mother,because she has been nothing but good to us. When BF and I met her together (in a restaurant) she was nice, respectful, complimented my necklace and wished us luck.
BF said that when they split up, she was normal, cooperated, and there were no problems with her.
A few months ago I met her on the market and I was carrying bags to my car, she asked if I wanted help. I refused but still,compared to what I read here, she was nice.
She made the girls go to our home when thy refused,saying that they need to see their father. She said to them to be nice to BF and me. (They said to BF that their mom told them to be good and to talk to us,but they will not)
So I don't think it's from her,because she's really nice and really has no reason to be jelaous as she is the one who brought up the divorce in the first place,and I don't think she'd convince the girls to anything as she really does not do anything to us. She works in the same company as BF and she's very polite and respectful to him,and me if I go to his work (I do it sometimes).
But the girls have been like that,and he has been miserable. He doesn't show any affection to me, doesn't talk, doesn't do anything. He watches TV and looks horrible. He doesn't go out,talk to anyone,he isolated himself from anyone and anything.
I offered him to talk to me,he didn't want to. To go to a counselor,he didn't want to.
He's absolutely miserable and desperate,and not just that it annoys me that he won't talk to me (it's like I'm not in a relationship), but it truly hurts me horribly to know that he feels so bad.
What can I do?

PS. The girls live with their mother now. It was 1 week at mom's , 1 week at dad's but now it's all at mom's.
They tried to force them to go to dad's (even their mom tried to make them go to us at least for a day if not a week, yes she did, she brought them herself - and it's not to get rid of them as they didn't even plan to be in the house at the time so she is not the problem), but they were horrible that time,so they didn't do it again.