I worked on the film-- made the armour and many of the weapons......He wore a couple of different shirts. When he arrives in the past, he wears a pretty typical double pocket button down shirt. Later, he wears something different and it's the blue shirt I think you're referring to, but may be not. I remember the later shirt being a double breasted type shirt. A bit like a Civil War era cowboy shirt, but "rustic" or "medievally" because I guess the idea is that it's been provided to him by Arthur's people.
I've tried attaching the only image I could find quickly. Hope it helps.