Which name to pick for [name_f]Ava[/name_f] [name_f]Adele[/name_f]'s brother: [name_m]Christian[/name_m]/[name_u]Kristian[/name_u] or [name_m]Sebastian[/name_m]? [name_f]Do[/name_f] you know people by these name and what are they referred to as? I don’t really care for [name_u]Chris[/name_u] or some of the nick names for [name_m]Sebastian[/name_m] but like the full names.
I like both full names but agree that [name_m]Sebastian[/name_m] has more nn potential. [name_m]Seb[/name_m], Sebby, [name_m]Bas[/name_m], Bash, [name_m]Bastian[/name_m], [name_m]Baz[/name_m].
The Christians I know go by [name_u]Chris[/name_u] or their full name. I hope that helps with your decision. Good luck!
The [name_m]Christian[/name_m] I know goes by his full name. I don’t know a [name_m]Sebastian[/name_m], but I think the most natural nn kids would make would be [name_m]Seb[/name_m] or [name_m]Bas[/name_m]/[name_m]Baz[/name_m]. Similar sounding is [name_u]Tristan[/name_u] but I rarely hear of this name being shortened. [name_f]Hope[/name_f] you are helped some. 
I think if you call him by his full name, others will as well! I know a little ‘[name_m]Edward[/name_m]’ who goes by nothing but [name_m]Edward[/name_m]. I know a [name_m]Christian[/name_m] who goes by his full name and a [name_m]Sebastian[/name_m] who goes by “Sabby” and one who goes by “Bash” but I think you could definitely have him go by his full name.
I like [name_m]Sebastian[/name_m] slightly more of the two. Good luck
My nephew [name_m]Christian[/name_m] is always called [name_m]Christian[/name_m]