There are 227 rules of conduct for Theravada Buddhist monks, some them very minor, such as not having your robe washed by a nun unrelated to you, or accepting a rug made of black wool. (The Buddha on his death asked the minor rules to be relaxed; unfortunately, no one could figure out which rules were minor and which major, so nothing was changed).
Basically, ordaining as a monk means giving up life as an "ordinary" person, or putting away personal desire and personal comfort.
So it came as something of a surprise to find on the Bangkok subway that monks are a preferred class to whom healthy, "ordinary" persons should relinquish their seats. Unless the monk is ill, elderly, or otherwise incapacitated, I would assume that he should not be sitting while others stand and that should he need to sit the floor would suffice.