If my husband cheated on me so I left the house, can he legally keep me from coming back?

I am so sorry for your situation.  I am worried for you.  Where is your son?  Is he with you or in the house with your husband?  Are you renting the house from your father in law?  You need to seek legal help in your area as soon as you can.  Try legal aid or women's services.  I can not really tell what the situation is with the hosue to let you know if he can keep you out.  If you are renting it - even if it is from his father - then he can not keep you out of the marital home unless a court allows him to.  But you have to get legal help here.  good luck.