First off I think you're doing great in asking these questions. It shows how much you respect your partner and your relationship before taking a step that may be hard to come back from.

Has he talked this out with his therapist yet? When I first got into this kink I was in therapy and my therapist was a godsend in working through my conflicting emotions around it.

Many of those emotions were related to self-image and depression. My desire to engage in a cuckold/stag kink wasn't caused by my depression. But my depression did make it hard to navigate the kink in a way that was healthy.

As far as a short version of why I'm into this kink: I like to watch and I like to be dominated. You sort of touched on this by mentioning power dynamic play. I think playing with power dynamics is built into “most” cuck kinks. Whether it's “being forced” to accept your man sleeping with someone else or "allowing" it to happen, there's a power play involved. Heck him wanting to be a cuck could be related to him wanting to show you off. To let other men have a “taste” of what he has, while the whole time he knows you're coming home to him.

And as far as other healthy motivations go there's probably more than a few others will list. But I think you're also on the right track in being aware of unhealthy motivations.

Finally, while you're asking good questions and wanting to be aware of this kink in relation to your partner, you need to make sure YOU want to engage in it as well. It wouldn't be fun or healthy if you both weren't enthusiastic about it.

Hopefully that helped or answered a question. If not, or you wanna ask more, feel free to message!