Oh. I know the manga as well. But I don't remember the name.
The "father" was some kind of agent or so before he got the kid. And the "son" found a guy who lookes like hs dad and has an affair with him later. But when the guy found out that he looks like his father he backs out. The last I read was that hey have to flee to another country, because some of the "fathers" past have found them.
But shit. I can't remember the name.
But I found it again.
http://www.mangago.me/read-manga/papa_s_assasin_ryuunosuke_wa_tonde_yuku/
Thank you so much!!!
You're welcome. ;)
Summary: In short it’s about this man who I believe finds a boy or someone gives him a boy and he adopts the boy as his son , but the boy is in love with him, also there is a threesome between the boy and his two friends one girl and another boy, I don’t know if that maybe helps?