Most trainers are stupid and uncaring about whats best for the kid, and only care about whats easy for them.

A southpaw is gonna have an advantage whatever way he fights but its still best to stay southpaw, unless the fighter is short and stocky with short reach then it may be an advantage to go orthadox because its easier to get inside against other orthodox fighters. But Manny pacman still manages well as an inside fighter, so I would suggest keeping him southpaw.