This need for kink is part of his sexuality and who he is, and it is a need. Just like you need to have another man in your life, he needs whatever those fetishes and sexual interests are. Nothing wrong with either.
He has been able to contain/control what he needs to thrive and be fulfilled, but kink will always find a way out. He sacrificed his needs because he loved you. After 12 years of holding that in, this crossroads was bound to happen. Sometimes love is just not enough. As you stated, basic conversations turn in to something kink related. He has no outlet, and despite you giving him the consent to find one via whatever means, he does not want it. For some people BDSM and such are emotionally tied just like lovemaking. Do you know how people reconnect with sexual intimacy as a physical expression of love? Fetishes and all of that may require an emotional connection for him, and if your husband does not feel that emotional connection with whoever it is, it will probably defeat the purpose and not be much fun for him. The thing is...he is mono, so meaningless sex or play partners may not be appealing to him.
You two are hurting one another. Honestly, some counsellors will tell you that when you realise a sexual incompatibility of that magnitude is present, the relationship should never have gone past the dating stage. Fact is you two are not sexually compatible, and there is nothing wrong with either of you. You like what you like. He likes what he likes. Unless there is a happy medium or compromise, it is not going to work.
As far as a therapist, you might have to look outside of the area. We found six counsellors, therapists, psychotherapists, etc. who specialised in alternative lifestyles, open marriages, poly, BDSM, kink, etc. It took work, and the first few appointments were conducted via video conference because she was on another continent.
I am terribly sorry. I wish I knew what to say.