What would the father of the groom's husband wear?

That's a great idea! We could alternate the tie color so he's technically not matching.

His your fiancé’s dad a part of the wedding party? If he is, but his husband is not, I would have his husband wear whatever suit he would like. He could pick something that is cohesive with what his husband is wearing without having to match exactly.

Assuming that the dad's hubby is not in the WP:
He can wear whatever he wants, there are no hard and fast rules on this . Both dads and thr step-dad don't have to match or complement the groomsmen even though it's nice if they want. But ... This is not a requirement.
In your situation: I would ask him if he wants to match the Groomsmen, to complement them'or if he wanna wear a suit of his choosing.

If YOU WANT him to match the dads and groomsmen,' that's okay but you should ask him for his opinion. He might want to wear something else, you never know.