Thanks for writing. So sorry your grandson had to endure this abuse. It's good he's in safe hands now.
Before I make any guided imagery recommendations, I'd like to state the obvious (which you probably already know, but just in case): This boy needs to be in therapy with a skilled practitioner who is well acquainted with posttraumatic stress and childhood abuse.
And because of the extra dose of guilt and shame this boy is carrying, it would be especially good if he could also be in a group with other kids who've suffered similar experiences. Nothing cuts the shame like sharing with like sufferers.
I also hope you've consulted an attorney, to see what can be done to keep these perpetrators away from him and from other kids too. Is custody still up for grabs? Does he in fact have to go back to that household?
It would be good if you could do something to show your grandson that there are legal protections that can be brought to bear. He needs to feel that he is safe now.
As for what he might use: he does have PTS. Depending on his maturity level and facility with words, he could either use the kid audios or the ones for adults. All things being equal, I'd suggest Shambala Stress Management for Teens and Bodhipaksa's Mindfulness Meditations for Teens and my Healthful Sleep audio.
I hope this helps. Wishing you all the best.