He likes his big brother's bestfriend (he is very young at the start of the story) because he seems nicer than his brother. And one time, his big brother's friends are at his home and it happens that they are looking at a porn tape but his big btother's bestfriend is at the back and is not as interested as the others about the video. He enters the room were they are and sits with his big brother's bestfriend, and has a reaction^^' So his big brother's bestfriend helps him to masturbate and then after many years they are together and in love...
Do you know the title of this yaoi manga please?

is it 'Anata ga Oshiete Kureta Koto' ? the 2nd chapter revealed like what you've explained. Though, idk whether it's the one you are looking for.

[img]http://i67.tinypic.com/2vbook7.png[/img]

Yes that's the one thank you sunose