My son is 14 years old and he is worrying about his penis size (13.4cm). how much long should a 14 year old teen's penis size? .

Long enough. Penis really does not need to be very long to serve it's purpose. There is a lot of variation in size at around that age. 13 cm is long enough. As long as he has developed normally in other ways - muscle mass, body shape, deepening of voice, enlargement of testicles, I would not worry.