[Solved] Contact issues

I need advice for my husband and his son (my step-son) aged 12.
SS was abandoned by his Mother in Sept 2014 (ten years post-separation) at v short notice and we were told he had to live with us in a letter from her solicitor. Up till that point SS and his older brother had 2 years of very stable contact arrangements under court order (by consent) in a rigid alternating weeks pattern, whereby they spent one whole week with Mum and one whole week with us, equal care, shared residence etc.
SS has had little (virtually none) contact from his mother in the 7 weeks following. We have corresponded through her solicitor asking her to set out new contact arrangements for him to see her and his little half brothers and his wider maternal family. None of his personal things have been sent to him from his Mum's house, none of the questions we raised through the solicitor have been answered. The solicitor has now deemed the matter closed - consent was withdrawn by the Mum, the matter of where SS lives is therefore settled as far as they are concerned.
In the weeks leading up to the rejection by his Mum, we had received an increasing number of distressed phone calls from my SS about rows he was having with his step-dad and Mum. The threat to him was that he would have to come and live with us because of these rows. We kept saying to keep his head down it would all blow over. When SS was evicted from Mum's house we were told it's all our fault for interfering and SS living with us is clearly what we want. It isn't. We told the solicitor so, we have never changed our view that what is best and what the children want is to have equal contact with both parents. We have never waivered from this view.
There was a court order put in place 2 years ago in order to get equal time and care of the children in accordance with their wishes, which the Mum did not want but ultimately agreed to; and because mediation failed on many occasions, often due to her withdrawing from it.
My questions are these:
How is a parent allowed to abandon a child and persist in failing to respond to any questions?
How can she be held accountable?
But more importantly, in what way can my SS get to see his Mum and wider maternal family?
He misses his little brothers and his cat dreadfully. He wants to see his Mum. She has not called him, but she expects him to pick up a phone and reach out to her.
She reacts to any and all communication we try on SS's behalf, mostly by doing nothing or the opposite of what is asked. Following the court order 2 years ago we had pretty much stopped trying to communicate with her as she refused all and any requests for flexibility or changes to the arrangements.
The older SS has continued the week by week arrangement but is not allowed to ask for her help when he is with us and vice versa.
Social services were only interested at the beginning in whether we would take my younger SS in and he would be safe. Is this really all a little boy can expect who has been ripped out of his home with his Mum?
Any ideas? We have already tried to give her time to come up with a plan or to reach out to him. We have received nothing. We have notified the court of the change in living arrangements but have heard nothing since.