This page has gone strange but I'll post anyway.
I think if you were making a scarf a totally black one with no fringe would be the best given he chooses to wear black all the time.
However, other thoughts are, a cross stitch book mark that has a Greek Orthodox religious symbol on it. You could readily make a chart to help you do this. You could even make a bookmark in knitting (in a very fine yarn) but in this case perhaps white. But you would need to seek advice on that.
I would not make any undergarment items. You could also perhaps make a black cushion cover or other colour that you know suits religious symbology and put a cushion inside the cover. Such things make for lovely gifts. A navy blue here may be nice. Perhaps look up some Greek Orthodox sites online and see what colours items tend to come in.
It's wonderful he was so helpful and you want to show him your appreciation.