Don't play her game - you should have a visitation order in place. If not go to court and get one. A pissing match will only harm your son.
X2.....and buy him a pre paid phone so he can call you and you don't have to deal with her any more than necessary. Oh and if it were me I would pay her in unrolled change dumped in a 5 gallon bucket every now and then just to prove a point and because you can!
