Davey is the odd T-Bird out
The fact that he rides in a sidecar instead of having his own motorbike suggests that we're supposed to believe he's under 16, as does the fact that at the end of the movie he willingly starts dating a girl who's been portrayed as not yet being in high school. (He doesn't look under 16, but none of the other T-Birds look young enough to be in high school either.) This begs the question of why the other T-Birds include him in the first place. Is he maybe the younger relative of a previous T-Bird who's graduated? He doesn't have the same last name as any of the T-Birds from the first movie, but he could be a stepbrother/half-brother or a cousin. Some of the rehearsal scenes show him looking at the others for approval of his performance, which suggests he's not secure in his position among them. Michael is singing his part of "Love Will Turn Back the Hands of Time" from the piano at the talent show
The song was improvised on a collaborative effort.