I guess my concern would be why would he think your son would need someone specializing in trauma/DBT/borderlines? Has he done therapy with your son? Does he think your son has PTSD or is borderline? I would take a bit of umbrage that he is diagnosing my son without ever having met him! You want the best therapist for your son, and it seems one specializing in children and who is a generalist would be a good first start, not a trauma/DBT specialist, unless your own T knows those are your son's problems. Can children even do DBT? How old is your son? Anyway, I would bring this up with your T and asked why he thinks your son needs a therapist who specializes in those issues?

If this is merely a way to get you to talk about DBT yourself in therapy, it seems really convoluted and not direct at all, just game playing. Do you really think this was his intent? It seems like T's want to model direct communication, not game playing. So I'm not sure I would suspect this of him since he must know how to communicate better than this. Doesn't he?