I totally agree with Sandman. That is not ADHD behavior. He needs to see a child psychiatrist. Has he had any recent traumatic events inhis life?
I read your post and perhaps he might have ADD, however the hearing voices and seeing things is not a part of that. Did you ever ask him what they look like or to discripe them to you ? Perhaps he is seeing a ghost or a spirit. I know it does not sound like the logical answer, however, when my daughter was young, she would stare up into the window going outside and sit and talk to it. I later found out that it was a ghost she was talking to. I know it sounds kind weird, however children under the age of 7 generally see these things. Where as adults do not normally see them. You might want to ask your child what kind of things they say to him. You might want to get the house blessed. Perhaps get a Saint Anthony pendant blessed and put that around the child's neck. I would be careful how you approach this. As most people do not believe in ghosts, however, I assure you they are very real. My daughter had a shadow ghost follow her everywhere until we moved and thankfully it did not follow us there. But it is possible, and these things do happen. Good luck, and I hope all goes well for your family.