Amanda,
I have three sons.  When things like this happen, I go into the room and first look around to make sure they haven't picked up any demonic items from a gumball machine, etc.  (Once this happened because my son had gotten an egyptian dog/god looking thing from a gumball machine)  If you find anything, throw it away (I don't care how much it costs)  Ask God to show you if there is anything in his room or in your house that could be attracting this attack.  After I get rid of the item, or if there is nothing visible that I can find I go to the room and take authority over any dark forces.  I bind them, I rebuke them and I send them out the window to the footstool of Jesus.  (He says in Hebrews 10 that he sits at the right hand of the father waiting for his enemies to be made a footstool.)  It helps to have your son also to do this with you.  It is important that your son understands that through JESUS, he (your son) has authority over these things and that they cannot harm him because of the ARMOR of GOD that he wears and never takes off!  My sons are home schooled and today I copied this from one of there workbooks (now I know it was for you!)  :

SECURE
In God's whole armor and His
 might, I am secure both
day and night.
In His truth and righteousness,
 I am kept by His
 faithfulness.

It might not be a bad Idea to put this on his wall where he can see it every day.  See, the demon was allowed to act demonstratively in his room, but it was not allowed to actually harm him!  PTL  
Also, one last thing that I recommend to do.  If it was a toy or something or a borrowed video game that attracted this, have your son repent (even if he unwittingly had the item) and in Jesus' name cast the image of it from his mind.  I also ask GOD to replace it with images of HIM or Heaven or just a place that they boys like.  

I have had 2 of my sons have "vision" type attacks simultaneously at bedtime right before God revealed a major truth to me.  It could also be that.  Nonetheless, your son was "slimed" by a demon and you need to just take authority and not put up with it because WE HAVE POWER OVER IT, IN JESUS' NAME!!  

Happy, fruitful praying to you!
Deborah W.

PS
What a wonderful lesson both you and your son are learning about your AUTHORITY.  May you walk in your AUTHORITY always!