Koisuru Barairo Tenchou

From Bamboo Feathers:

When he was a child, Isao was inspired by a florist named Aiko and fell in love with flowers. After saving his father's company, Isao finally leaves his desk job to realize his lifelong dream of owning a flower shop, opening it in the same location that Aiko's shop used to be.

Right before opening, he finds out that Aiko had passed away when he meets her widower Koutarou and their young son. Koutarou is utterly devoted to his son Zen and carries on the love of flowers that Aiko instilled in him. Thus the new florist Isao falls in love with Koutarou, and tries to win his heart through the flowers they both love.