Son's therapist wants us in family sessions - ex refuses

My 11yo has been seeing someone for separation anxiety and the therapist recommended both parents attend some sessions. Makes sense, right? We told her we'd try. Except my ex straight up said no, something about "it's not beneficial and wastes time." I think maybe she doesn't want to be in the same room as me. Kid definitely needs this but I can't force her. What are my options here?