[Advice to a woman whose son has been severely handicapped due to encephalitis he had when he was five. He has seizures and the woman believes her son is possessed by demons]

It does not say in the Bible that demons cannot posses Christians. I wish I could remember the verse, but there's one that says something about a spirit in someone after having the Holy Spirit in them. I'll explain more..
Sickness is always demon attack. Not everyone believes this, however I had "ADD" and a lump in my breast. This pastor said that ADD was a weak spirit and cast it out. He also said something similar about the lump and that's gone. It's what I believe and have been taught about sickness.
Your boy can be healed of all his sickness.
"By His stripes we are healed." That verse doesn't stutter.
Going by the logic that Jesus died so we can be healed, and the logic that sickness is in fact caused by demons.. I can't see what that could be but demons causing that.
We have power, through Jesus, to cast that out. Try to pray against it in Jesus name.

Remember, we are not battling the flesh, but of the spiritual relm.