Nicknames:
Seb, Bas, Bastian, Sebby, or maybe Bash if your kid turns out to be a little bruiser.
Famous people named Sebastian:
Actors Sebastian Stan and Sebastian Shaw; football players Sebastian Vollmer and Sebastian Janikowski; Sebastian Bach (both the Skid Row frontman and the classical composer, whose first name was actually Johann).
Fun fact:
Bastian, a common diminutive of Sebastian, was the name of one of the main characters in The Neverending Story, which is an awesome movie that you should definitely watch with your little Sebastian someday.
More Inspiration:
Solid S Names For Baby Boys, Liam, Alex, And Beyond: Boy Names With Natural Nicknames, Perfect 3-Syllable Boy Names, Celebrity Baby Names Of 2018,