Mayonaka No Oyatsu summary:

Fuji Azuma is thrilled to have the chance to spend two whole weeks with his adorable 8-years-younger nephew Taira, head over heels for the kid and the proudest uncle there ever was. But when he learns at the end of their time together that Taira and his mother are moving overseas, he's shocked to receive a parting kiss and a vow that Taira won't return "until I'm taller than you, Azuma-san." Four years later...and Taira's back to make good on his promise.