Revealing mistake: In the closeup of Tina's dead body, her chest moves. (00:44:00)
Revealing mistake: When Anita lies dead on the ground with her throat slit you can see her pulse in her neck. (00:53:05)
Revealing mistake: When Ethel gets a cleaver in her face, blood spatters everywhere. Yet the cleaver blade is still clean. (00:56:35)
Revealing mistake: When Jason is raising the machete above Tommy in the woods, it's obvious that his eyeholes are covered with black cloth. This also happens at the end of the movie, where Tommy hallucinates about Jason in the hospital. (00:04:45)
Revealing mistake: After the "Friday the 13th" logo explodes, the supports that were holding up the model are briefly visible through the smoke. (00:05:40)
Revealing mistake: As Junior is riding around on the bike, right before he is murdered, you see the same clip of him turning and riding onto the deck twice. (00:55:30)
Revealing mistake: When Tommy begins to beat up Junior, at the trailer park, you can see that it is a stunt double (who has darker hair) replacing Tommy. (00:49:50)
Revealing mistake: When Demon is using the outhouse, his pants are unzipped but not pulled down. (00:53:00)
Revealing mistake: When Vinnie has a road flare shoved into his mouth, it is obviously a dummy as the road flare is inserted (his skin is much paler than in previous shots). (00:24:30)
Revealing mistake: When Pete gets his throat slashed, you can plainly see that there's no actual cut, it's just a line of stage blood. (00:26:30)
Revealing mistake: Right after he gets off the tractor, Reggie's clothes appear to be dry, but he was out in the rain.
Revealing mistake: Violet has blood on the right side of her shirt before she gets stabbed. (01:05:10)
Revealing mistake: During several of the deaths committed by the impostor, look closely. The shirt sleeves are dark green instead of the dark blue sleeves of the jumpsuit the impostor wears. Clearly Tom Morga, the hallucination of Jason, did these scenes instead of Dick Wieand, Roy Burns/impostor Jason.
Revealing mistake: When Tommy is about to slice Roy's arm, you can see that he is not really holding the machete in his hand. (01:19:20)
Revealing mistake: When Pam is out looking for everybody, when they show her inside the truck, her hands don't appear to be on the wheel.
Answer: Several possibilities: 1 The mother requested he not make any contact, 2 They could've moved away, and he lost contact until then, 3 He may have been drafted into the army, and she could've divorced him during his time away and moved into whatever state this is located.