Probably about 3 years. Which would look severe as children (top in HS and bottom last year of middle according to Korean school system which would be freshman HS and senior in HS according to Western standards) but less apparent when older since they are now freshman and senior in college.
His mom loved him. I think there's some point in novel where uke meet ye doyoon and he told uke, he actually finally be happy to meet his mother again. But uke wasn't satisfied with it so he sought revenge for ye doyoon
GODDDD THIS WAS SO HOT