Is this a scam? (CSGO)

Can't tell you if it is or not in fact a scam. But if some random user added you just to trade and it wasn't at least done through a trading service, then I would approach everything as a scam.

He might have wanted quickswitch, or it was gamer's girlfriend/mom angry at him, so she decided to screw with his csgo account...

what's quickswitch?
you get notified when the other party changes the deal..

They can't scam in the trade window. Just make sure you check what they're trading before you accept it.

most times they have 2 same weapons with different qualities on inventory, usually scammers will promise you a very expensive weapon but give you the cheap lower quality one
always check weapon quality during the trade and never let people rush a trade because otherwise you won't have time to check properly
in csgo trades i always check the item market page in-trade window, if for some reason there is a wrong quality it's insta-block :)