Under His Heel by Adara Wolf

Alex is trapped. His life has been one bad decision after the other, and now he’s forced to work off his debt with a spaceship captain or risk a fate worse than death.
Captain Tracht is respected and well-connected. Unfortunately, he also has some very questionable private interests, and makes no secret that he intends Alex to participate in them, whether he likes it or not.

It’s an experience of pain, humiliation, and tears, but somehow Alex finds himself growing less and less interested in his eventual release…