Responses
Im pretty sure its this one
http://www.mangago.me/read-manga/winter_that_year/
Is it this one? http://www.mangago.me/read-manga/ai_ni_chimayoe/

It was a story about the main character being completely in love with his best friend who ended up getting married to a woman and they later got a son together. The son of his best friend ended up imprinting on the MC in his childhood which persisted even up until he was a grown up.
The main character has visibly aged with wrinkles around his eyes and all that but the son who is now around 20 yrs old is still chasing after him! It's a somewhat short story about a young handsome persistent guy head over heels all throughout his life for his dads best friend who is always reminded of his best friend when he sees his friends son.