So....he's a Q. The father's voice gives it away even if the episode itself was very coy about who Trelane was. It's clear this is Trelane from after his encounter with the Enterprise crew in TOS and that's why he tells Spock he's always amused by his behavior. Older Trelane is still a petulant troublemaker and the rest like him (*ahem*) clearly don't impose enough parental discipline on this guy. This now makes twice he's interfered with the same Federation starship, chastised and had to be called back inside by his family. 
