Honestly it could be so annoying at first sight. But do you really need to spill out to him about his manners? Yes o. There are actually some people that need to be reminded. Do you know this guy might not see any big deal in it. He might simply tell you "I was just stretching my legs" Lol.
If it was my seat, I'll just look straight into his eyes, point my fingers towards his legs. Shikena. Power of action. I don't need to talk.