I would assume that your son is getting this notion from G, and I'm also assuming that G is a little kid about your son's age. Your son is young, so you still have a good deal of influence and can offer him both authority (knowledge, Wisdom, Security) and positive guidance. I would start by suggesting he make some real friends and forget about the occult, because it may lead to no good (explain to him what it is and how dangerous it can be). I would also go into his room, when he's not there, and have a talk with G. Tell him that it's time to move on; explain to him that he is dead, and that there are lots of other wonderful places he can visit with lots of new friends that are waiting to play with him. Finally be explicit and tell him that whatever he decides, you don't want him to talk to your son or to be in your house. If this still goes on, be prepared to go back into your son's room, when he is not at home, read some Psalms, say the Lord's Prayer and sprinkle some holy water. It might just not be a little kid you are dealing with here, and you and your son may be deceived. The idea of an imaginary friend could be a valid one, but since I see that you are deeply concerned, I tend to dismiss it in this case. After all what little kid, dead or alive, knows about seances? Where did this dubious idea come from? Be alert to any possibilities.