What is "reasonable" amount of phone calls to my son from my ex husband?

How often do you talk to your son every day? Would it be reasonable if you were only able to talk to him twice? Don't let it bother you. Let the dad call him as much as he wants. If the son wants to talk let him. If he doesn't want to then don't make it. Let your ex know that you appreciate the interest but you want your son to be happy and not felt like he is made to do something. Ask him to back off for a little so your son has the chance to miss talking to him.