@Ceiye basically, Ros used up all his mana midway through Season 2 when he sealed Rchimedes' magic as they left the Dimension Rift. But, he can still use magic if he has a Mana Maker like he makes in Season 3 and he uses during the health examination part.
So the question arises, why doesn't he just always wear a Mana Maker just to be safe?
Leave him alone, it's a peaceful era now, he doesn't wanna fight....