I asked my husband more about what he means by being 'punished' by not being into spanking. He explained that Mike gets to do something that has been a part of my sexual fantasies since childhood and he feels like he is missing out on something or some part of me.
He regrets his choice not to take part in spanking though you gave him the choice. The question becomes, should you re-invite him?
SC made a good point that every partner is going to be unique, but such uniqueness doesn't diminish the value of other partners. It sounds like that concept needs to resonate with him.
That being said, you could engage in spanking with your husband during your private time and continue with Mike as well. There's no rule saying you can't or shouldn't. You should probably first decide if that was something you want. Do you want the responsibility of teaching him about it? Would doing it with him be healthy for you and for him? You may also want to talk a bit to find out if he's just doing it out of jealousy, or because he really wants to be closer to you. Only you can decide, based on how well you know him and your level of trust in what he's saying, whether it would be a good idea.
I would think you would need to make sure he understood, though, that even if you did decide to guide him into the kink world (at whatever level the two of you are comfortable with) that Mike isn't going to magically be obsolete.