Matt stood up grogilly. He looked around, saw that the others were still fighting. He also saw that nobody had seen him recover, and no one currently percieved him as a threat. He noticed Trevors attacking RH Irvine, raised his blaster out of his holster, aimed at the lizardman's head, and shot three times.
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.