BDSM is very emotional and physically exhaustig. My wife has two DOM's in her life. We either have to "play" with her together or take turns. Your mind and body can only take so much. It will be very important that everyone knows their limits.
Being poly and kinky can be tough but so worth the e#ort when the pieces come together.
Don't give up on him we were all monogamous at some point, it's the same for kink everyone starts vanilla. Keep communicating with him, keep expanding his exposure to both. With time love and...