He sounds like he fits the classic profile of a sex addict. Has he had this diagnosis yet? Characteristics of sexual addiction here. Ten Types of Sex Addiction here.

It sounds like your significant other's sex addiction falls under transactional sex addiction, due to the power and risk involved:

Trading sex: Using sex as part of a business transaction. This is addictive because of the risk, cost, or repetition of early trauma

Paying for sex: Purchasing sex as in compulsive prostitution or phone sex services