((Oh yeah, I guess you wouldn't be able to buy it like in real life, where it'd be a semi-old antique, not an ultra-old like it would be in the SW Universe. Let's just say he made it out of materials he found and bought, then painted it green.
And guys, y'know Matt's right there, right?))
Matt: Uh, guys, I'm right here. And for a weapon, I have a blaster.
*Matt took out the blaster*
Matt: Its safety's on for now, and I've practiced a bit with it. I think it'll be useful.
Vader's wife was very pregnant when she died 19 years ago. All of a sudden a 19 year old who is very strong in the force and has a distinct resembalance to him in his younger days rolls in from Vader's home planet with his old Master Obi-Wan (Who was the ONLY other person preset at the time his wife died. And to boot, the kid's last name is Skywalker.
So in answer to your question, he knew Luke was his son because his name is Darth Vader, and not Darth Retard.
-Forum post on why Vader knew Luke was his son.