Why would it be bad for your son to attach to his therapist?

You're saying that your goal for his therapy is not to compensate for his lacklustre relationship with his dad, but:

1) to what extent do you really get to set the goals for his therapy?

2) it seems highly plausible that some of his anxiety might be rooted in an insecure attachment to his dad.

3) yes parental transference can be painful and yes kids experience it, but sometimes what comes up in therapy is painful and try as we might, we cannot shield our children from that.

I am not saying that I think you should have your son do therapy with this fun guy or anyone else. It's a big decision to entrust someone with your child's emotional well-being and it's just one way to help kids with anxiety. I am saying I'm not sure I follow your reasoning on this.