That was incredibly good. Really. I've read a lot of Harry/Sirius and Sirius/Harry fics, but so many of them are pwp that make no sense. I love the fact that there's some guilt and awkwardness 'cause of their relationship as Godfather and Godson. I think that's an aspect that people just don't write believably most of the time. Yours felt very "right."
If you ever write a longer fic with these two, let me know. I like making "covers" for fics I particularly like, and you've got some serious inspiration there!