My brother is 5'6 and always had a million girls interested in him because he is funny, confident, and kind.
My husband is 5'7 and while I obviously didn't observe his dating life before we met, he did get married.

I don't know how to say this without saying obnoxiously stuck-up but a lot of me pursued me, so it's not like he settled for someone who was undesirable.
So there are two stories. Don't stress about it. A lot of people who are great just don't have many relationships. He might need to work on his social skills and personality, but unless he asks you for advice, I wouldn't tell him that.