How to refer to someone if he is younger than I (but not knowing his name)?
A bartender is basically a stranger, calling him 兄弟 is a bit too close

'小哥' would be more suitable (not 小哥哥, that's for kids), you are not saying he is your older brother, you are saying he is a big brother of someone.  

'姐姐' mean older sister, but you still call woman who is younger than you 
'小姐' 

Calling a kid 小哥哥 (litter big brother) or 小姐姐  (little big sister) is also acceptable, you are not saying he or she is your big brother or big sister, you are stating they are someone's big brother or big sister. 

In Cantonese, we can also call people much younger than you '姐姐仔' or '哥哥仔',  (a way people from older generation show affection to people from younger generation)

If a ten years old boy takes his four years old sister to a candy store, it is perfectly fine for the store owner calling the boy 哥哥仔(小哥哥)