The closest thing to your request I know is this https://www.mangaupdates.com/series/oy8fmvm/the-youngest-has-many-secrets (the manhwa is not on mangago yet but it's on other sites until ch.19), it's still a child FL BUT she cross-dresses, so for a good part of the story (it should be until about chapter 40 of the manhwa?) the family believes she's a boy and treats her as such, but still loves her the same as other childcare stories. The reader knows she's a girl but if you forget about it she's actually a bratty little boy being doted on. IT'S SO GOOD!!
Otherwise there are these 2, some of my absolute favorite korean stories from the same author, but they're novels and partially translated or not translated:
https://www.novelupdates.com/series/counts-youngest-son-is-a-warlock/ (although he's not adopted, the story is family centered and he's very doted on due to being the youngest and due to a traumatic event he went through as a child)
https://ridibooks.com/books/3586030607?srsltid=AfmBOopGPKTl2A4yM_QuBoFKsabnmZAK0yyUuErYDzGvtmBmGLE1NkN_ (not translated, also family centered and here he's not adopted BUT he's a soul that transmigrated into the son's body. Yes, they find out and still accept him)
***you can find the korean raws for both on bookt*ki if you can stand MTL***
https://www.mangago.zone/read-manga/i_ll_retire_after_saving_the_world/ this is also PEAK!! It's not of the rofan genre and the mc is an adult, but it's centered on the relationship between the male lead and his adoptive father
https://www.mangago.zone/read-manga/stardust_family_official/ this also fits every criteria, and it's also one of my absolute favorites, but... sorry in advance lmao
I actually have a whole list of stories with male mcs that are more or less family/found-family centered if you want, although those 5 are the ones that fit your request the most
https://www.mangago.zone/home/mangalist/2654897/?filter=&page=1
Yes I know, there's not much unfortunately :/
Does anyone know a manga or webtoon where instead of an adopted daughter trope its an adopted son and he is the main protagonist? I have noticed when its an adopted son they are not the main character its the mother that adopts them. I was just curious are there any stories like Male Lead's Adopted Daughter or The Saintess Became the Northern Grand Duke’s Daughter but son.