How best to help our adoptive son?

My husband and I are foster carers and, just over 3 years ago, took in 3 children from a very troubled, abusive background. They are now 8, 5 and 3 and we have started adoption proceedings. Our 8yr old is the only one with any conscious memory of his past but most of the time he seems to have adjusted well and is happy and affectionate. However, if one of the younger children does anything naughty, or has a bed-time accident, his immediate, instinctive reaction is to say "it was my fault" or "I made them do it" and then kind of brace himself as if he's expecting a blow. He snaps out of quickly but then gets upset sometimes and talks about his "bad mommy." We've explained many times that its okay and bad mommy's gone now, and that they can stay with us forever and ever, but it's so heartbreaking because it means he must still have such bad memories. Time and love we have in abundance but should we also consider proper counselling for him? We just want to do everything we can to help him.