I've read japanese manhwa that has the exact plot like this one, failed first crush - keep being friends - first crush has a son - the son interest to MC - figure that the MC had a feeling for his dad - try to seduce the MC and BAAMM being the boyfriend of the first crush's son 
At least the japanese one show the age difference so not to much guessing, but this is hmmmmm
Lets see how it goes
At least the japanese one show the age difference so not to much guessing, but this is hmmmmm Lets see how it goes