Welcome to our forum.
I don't have much to add to the advice already given. Obviously you have a lot on your plate right now. You'll have to decide whether the added complication of polyamory is something you can deal with right now.
Can I ask, how old is your son? Is there another therapist he can see? What would your husband like to see in a therapist?
My sympathies go out to you, you have a hard situation to deal with. Hope we can help somehow.
Love means never having to say, "Put down that meat cleaver!"