The child is probably confused, hurt and angry. I do not know why he is not able to live at home but whatever it is it has upset his life
I completely agree with this. I've worked with many looked after children, in both a care setting and those looked after by family. The overriding feeling towards their parents, for the vast majority, is anger. Even those who have regular contact with parents, or those that have chosen to live with family for more stability. Even those whose parents are too physically unwell to look after them, the children still feel anger towards their parents for not providing them with a 'normal' family life. And those are teenagers, many of them having have had years to adjust and process the situation. This is a 9 year old, a young child. Even if he appears to understand and accept the situation, he's still so young.
It may be a lot for your DD and SiL to cope with and be upsetting for them, but to be removed from parents for any reason is so stressful for a child and can be very traumatic. I know you probably already realise this, and sorry if it's hard to hear but he needs time to deal with it in his own way. Do they have any form of support workers he can talk to? He's unlikely to want to talk to family. Also, from your DDs side she could probably do with some sort of counselling herself to have a space to talk about her feelings, even if those are frustration at her child not contacting her, she needs a space to work through that and ensure she's not showing her son that's how she feels. In the mean time, could she send him letters? He has the chance to open those when he wants and reply if he chooses to.