My 16 year son has a penis about 2 inches when not erect and about 5 inches when erect. why is it 2inches?? & will it grow ?? what should i tell him ?

2 inches when: not erect is not abnormal. Many penises are small when not erect. Nothing to worry about.

Grow vs show: Understand that one primary difference in penile presentation is the difference in how much they expand during sexual stimulation. There is likely some advantage to both, ie storage size vs use size. Some with larger size at rest and don't expand as much. Since he is basically normal you can tell him he is. Also ask him not to compare himself to porno guys he may see.(hired for size)