ANSWER: The Old Testament of the KJV Bible refers to ghosts eleven times and the New Testament refers to them one hundred and thirty-five times for a total of one hundred and forty-six times! Most New Testament references are derived from the Greek word pneuma (Strong's Concordance #G4151) which roughly means "spirit." The KJV errorneously refers to God's spirit as the "holy ghost" (instead of the more correct "holy spirit") one hundred and thirty-five times.
Surprisingly, one reference to ghosts involves Jesus' disciples using the word to describe HIM! Jesus had his disciples get in a boat and go to the other side of the lake while he went up a hill to pray (Matthew 14:22 - 23). Sometime in the middle of the night, as the boat was being tossed about by the waves, Christ stopped praying and began walking on WATER toward the ship. When the disciples saw him the screamed in terror "It's a ghost!" (see Matthew 14:23 - 26).
Reading your Email, it looks like what you might be calling ghosts are actually rebellious, evil spirit beings (demons). To determine if demons are involved, we suggest you first do some thorough investigating. Children of a young age many times do have imaginary friends. It is their process of maturing to become sociable beings. When they become older, they outgrow their imaginary friends and make real friendships. Make sure the child does not really see ghosts or aparition. If he merely talks to a friend, then he probably is playing a friendship game.
If, after all your investigation, the "ghosts" proves to be real, then demons are likely. Christ, in Matthew 12, discussed a situation where a man had an unclean spirit (demon) leave him for a while. When the evil spirit could not find any place to go, he came back and found his "home" (the person he had possessed) "empty and clean." This means, after the demon left, the person did not fill the void left by the spirit with something that would keep him out (e.g. God's spirit). Not only did the demon possess the man again, but seven of his evil friends came with him! This illustrates how serious a problem such spirits can be and how the situation can get worse if not dealt with.
In Matthew 17, the disciples came to Jesus saying they could not cast out demons from this son who would throw himself into the fire and water. This was a very difficult demon problem (Matthew 17:14 - 21). As the word of God shows, He said they did not have enough faith. However, He also said this was such a severe demon problem that the demon(s) would only leave by fasting and prayer.
Therefore, if you find this is a real unclean spirit problem, both your friend and his son should pray together. They, in the name of Jesus Christ, should ask God to remove the demon and have it never return. We, on our own power, cannot remove ghosts or demons. The Jesus of Scripture, however, can if you have the faith. If this does not do it, move. It is not worth staying there. Your friend and his son need a healthy environment to grow.