Thank you for your query at AskTheDoctor.com
I understand your concern.
An average size of an erect penis ranges from 4 " to 7 " ( 10 centimeters - 21 centimeters ).
So you need to first ensure that the size of his erect penis is lesser than that.
If that is the case, it could be due to low testosterone levels and serum testosterone needs to be evaluated. If the values are indeed low, then he can get testosterone supplements which will improve his size. Such deficiency is also accompanied by loss of secondary sexual characteristics like lack of facial hair, slightly feminine voice, lack of chest hair, etc.
Then there are other methods like Jelqing ( pelvic floor muscle exercises ) which increase the blood flow to the penis and subsequently increases its size.
So do not worry, meet your physician and get a detailed evaluation done.
Hope this was helpful.
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