"Hey doc, can I get out of here for a bit?"
"Sure. But you have to come back, we still don't know if you're in nominal conditions."
With this Ethan left. The creatures they had fought seemed to be vampires and he would treat them as such. Of the old superstitions their hate of UV light was the one he was most willing to believe so he would have to buy a weapon and ammo now. He headed to a pawn shop his uncle owned.
"Hey Al, mind if I buy a gun?"
"Friend of mine's in trouble. He's in the hospital and I need to protect him. I don't trust the police on this one."
Al looked suspicious.
"All right. Take this one. But if I hear anything bad happened with one of these I'm turning you in. Or your mother'll kill me."
He took the sawed-off shotugn, hid it and ran back to the hospital, and checked in with the doctor.
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.