Has he been diagnosed with Autism?
It could be that he has something else going on. It's not unusual for children to act out when their family life is disrupted. Have you taken him to a child psychologist and had him evaluated? The fact that you say he's very loving makes me think he probably isn't autistic or has a pretty mild form.
Whether he is autistic or has some emotional problem, he's going to need extra help, attention, and maybe therapy. The earlier it starts, the better off he'll be. To find out how to get more help, contact your local department of children's service or some other organization that would be able to point you in the right direction. In many places, a diagnosis like Autism can get him a special education aid in his classroom that can help him and his teacher.
I wish you all the best. You are doing a wonderful thing raising your grandson. He is very lucky to have a grandmother like you.