How much rent should I Charge my son each month for rent?

sevenhills

silvercar said:

If he did move out, what would you gain? If you charge him more than this, you may well struggle when he does move out. Then consider how much support he gives you around the house, plus his company; maybe you want to encourage him to stay with you rather than charge him a rate that pushes him out of the door.

I find lodgers are more appreciative of a room than your own child, the OPs experience may be different.
Look on spareroom.com and see how much a similar room would cost.