He is as hard-wired for non-monogamy as some are for homosexuality. He is young and his culture frowns on this, so he suffers with a sense of sin and has a wish that it were somehow easier to be his true self. It has nothing to do with any current relationship, IMO, and everything to do with All of his (many non-monogamous) relationships. He seeks understanding and acceptance he has (not yet) even given himself.
It may take him a while. I knew I was non-monogamous by age 14, but did not have my That's Okay epiphany until I was age 60. If you are a kindred spirit of mine, reading this and still struggling, please Know, it is Way Okay to be wired for non-monogamy.
Best to you and yours in all things.