For an Orthodox Bishiop's Visit, I'd recommend as smartly formal as you can. It would show a great mark of respect, which he is sure to recognise - even if a bit startled at first by a pair of men's knees.
If he's lived in your country for a while, then he'd be familiar somewhat with the kilt. If from Mt. Athos or somewhere, maybe not, but he'll be certain to appreciate your spick & span outfit. (He might want to wear one under his Cassock once he sees yours).