I am appalled at all the responses of "say nothing".

Asking him to clear up your legroom is neither rude nor aggressive. Just tell him you don't have enough room for your feet. You don't have to be rude, nor do you have to be overly polite under the circumstances.

If he does not resolve the matter, then either insist that he does or call the FA to resolve the matter.

You paid for the legroom. All of it. It's as simple as that. Letting him take your legroom would be like letting him take twenty bucks out of your bag.