You should talk to your pediatrician
about this concern. The first thing to keep in mind is that there is a wide variation between different children in terms of how quickly they progress through puberty. By the age of 15, there should be some signs of entering puberty, such as some growth of pubic hair and some changes in the penis. However, the earliest sign of entering puberty is an increase in the size of the testicles, and this is the thing that parents tend to miss. Your pediatrician, however, can examine the testicles and determine if they are increasing in size; if they are, then this is just delayed puberty and the chances are excellent that things will progress.
Your pediatrician will also be able to look for any signs of an underlying disorder. For example, one of the most common cause of delayed puberty in boys is actually another medical problem, such as inflammatory bowel disease
or a nutritional disorder. Fixing this underlying disease will allow for progression to puberty.
In those who show no signs of underlying medical problems and no signs of progressing to puberty, the next steps would be to perform some basic initial laboratory tests.