Murata kind of fucked up when he let Boros kick Saitama into outer space.
Well i always assumed that saitama was allowing himself to be moved by boros' attacks since he was trying to go easy on him.
Why would somebody not be able to move Saitama? He weighs 175 lbs max.
I believe that Saitama is capable of simply stopping himself from being moved. There have been plenty of times where saitama was hit with a powerful attack and didn't even budge. I think it entirely depends on his level of focus, and whether or not hes serious about the battle.
If anything he is still susceptible to anything that is sharp or peircing( The cat that scratched him and the mosquito that bit him.) When the lion monster was talking about gouging his eyes out Saitama immediately starts moving and kills him. When Sonic attacks with his sword, Saitama breaks it with his teeth instead of taking it like with the punches and blows he receives from almost all the enemies he's fought so far. So if an enemy appears who is as fast as Saitama and is holding a bladed/piercing weapon or fights in a manner that allows his/her attacks to be like blades, then Saitama will find the fight he's been looking for.
You could argue that saitama bit sonics sword out of sheer anger, but you are correct in that most instances of someone coming at saitama with a blade of some kind, even in the original webcomic, he's gone out of his way to avoid being hit. If it were a punch, he'd take it head on out of confidence. Perhaps he isn't sure how bladed weapons would affect him, and is simply acting cautious?
Did gases and other chemicals weapons can affect him?
No idea, it's never been tried. I like to believe they simply wouldn't work, or would just have a hilariously mild effect, like giving him a cold.