Yes, sit him down and YOU propose to HIM what you think his reasonable financial obligations are. Tell him that you're hoping you can handle this just between the two of you, but that you're prepared to do what you need to do to protect you and your son, make no mistake.
Have you put any pencil to paper as to what you think his obligations should be? Run it by a third party, too, as you're likely to NOT ask for as much as you should.
