Absent mother.

My grandson was 11 yesterday, we (him, his dad and me), had a great day. Cinema, meal then let loose in Smyth's toy shop to choose a present from me.
My son called me at midnight to say GS had gone to bed at 9pm, then came out crying & did so for over an hour.
He hasn't cried since he was a baby, which was a great concern to us, even when hurt he would put his hand over his mouth to stop himself.
He said his mum wouldn't let him cry. This is about the 5th time in the last 3 mths he has had a little meltdown.
Its all over the fact his mum won't contact him
He has lived with my son for 6 years now, in the early days she would come irregularly and occasionally answer the phone to him.
She always let him down. When he lived with her he was neglected, under nourished, dirty and left with unknown people. Social services were a nightmare and caused much harm.
They advised her to move away from us in London, she went up north. My son continued to visit every week, we watched our GS deteriorate.
He was so unhappy and never wanted to go back, he said things happened.
I wrote to his mother a heartfelt letter, pleading for her to let his dad have him. She said I was mad.
A few weeks later, out of the blue she said my son could take him. He took her to solicitor and had him signed over.
Our GS has had some sessions with CAMS but has never spoken about his time 'up north' but says his mums bf was bad to him.
She now lives with someone that has a child and is playing mum to her.
The year before last when she had little contact we asked her to at least call him every now and again, she did for a few weeks then stopped.
Even though we reassure him constantly he thinks its his fault she doesn't bother with him.
My son asked her to call and tell him none of this was his fault, after a week she did (she forgot and had to be reminded), promised to call and never did.
She won't answer her phone to us anymore, we were not rude to her and always accommodated her in seeing him.
She hasn't seen him for over a year now and he has started to have these little meltdowns. We feel helpless.
He is a lovely boy, she is missing out big time. He's happy normally and says he doesn't want to live with her or have overnight stays but he'd like to phone her.
I think yesterday being his birthday hit home, Xmas he was OK.
I think the best thing she done for him was handing him to our son, the worse forgetting to keep contact.
What can we do to help this little one? What do you say to a child in this position? Does anyone have experience in similar circumstances that can offer any advice please, we are getting desperate.
Sorry this is long but trying to give an idea of our perspective