I've seen this kind of shaman magic mentioned in another manhwa though I've forgotten the name (or maybe it was anime, or both?) They're basically using him as a kind of voodoo doll/spiritual replacement for the curses and bad energy of the people the gang boss is killing. I believe they are using semen because that is his life force (it carries his identity/DNA and the ability to create life, particularly in Asian shamanistic culture semen and blood is seen as being spiritually powerful). Normally you would use hair, though, because that also carries the person's soul/DNA, but I think they are using semen and the father's blood for a much more powerful spell. Not sure if the author will actually get into what the father is doing outside of the shaman magic, but this would normally suggest he is a very very bad man who is indiscriminately killing so many people that he needs a very powerful spell to protect himself from all of the bad juju.
If he’s using his own son as a sex slave and using him as a decoy for the bad juju, I’d say he’s definitely a very bad man.
I’m interested to know how the son (forgot his name) came to be the guinea pig and how this is all going to end.
I do believe it’ll be a happy ending though.
Why do they need his semen it's been a full year (still at chp30) but please spoil it for me