You are doing great. I would definitely have a word with her not to tell him if she is going to call, let her tell you (and don't tell him). This way, regardless of the situation, there was no broken promise.
I too think you are doing great. How wonderful that he is being offered free play therapy. As a single mom who has spent thousands and thousands on therapy, see that as the gift it really is.
Sending you big hugs. You are a great dad and it shows!!!