I, personally, think this was unethical. You are not his client, but you are the mother of his client.
He should've been more adult about the situation. He should've told you about his feelings and given you the choice to meet in person and see if things would be compatible. If that were the case, you could've made the decision to see him or not to see him.
He, also, should not have continued seeing your son as a client. He took advantage of you, even if it's hard to see it that way.
Sometimes the secrecy of having a relationship over the phone is so tempting. You don't have to get to know that person and you can fantasize about what that person might look like.
Just be careful!