Depends. It depends what stage of puberty he is in. If he is 13 and has just started puberty the penis may be about 6.5 cm (2 and 1/2 inches). That measurement is taken with the penis stretched (measuring from the pubic bone at the top of the penis to the tip). If there is a lot of belly fat the penis may appear smaller than it actually is. When puberty is further along at age 14-15 then penis gets longer.
Varies. Can be different if you are 13 vs 19. The penis is growing as you are maturing. What's average? Unless you are very very small or very very large, average is just what you have. Adult male penis size is usually between 5 and 7 inches. If you are concerned with too small or too large, speak with your pcp. Ask to speak with the doctor in private. An exam may be needed. Feel comfortable.