I have a 13 year old. Is he too young to use whitening products?

I have my pre teenage and young teen patients start slow ie.  Review proper hygiene, Sonicare toothbrush, whitening toothpastes, rinses.these steps alone will typically get a young teens teeth shinier and whiter.  Dull looking yellow teen teeth in teenagers are typically the result of poor oral hygiene and heavy plaque buildup.  These buildup deposits get stained with good color additives kids love to eat.we typicalldo not introduce any whitening bleach (take home or in office) until the patient is around 16.the pulp (nerve tissue) gets receded as the age goes up making nerve damage and sensitivity less and less likely as a patient ages.

Teeth whitening and Bleaching teeth can be used for any individual 13 years and up, however your dentist must do a complete comprehensive evaluation of the mouth, teeth, and surrounding structure PRIOR to having tooth whitening or teeth bleaching. Many dermatologists also prescribe doxycycline antibiotics or tetracycline family antibiotics for treatments on young adolescence which can cause color alternation to the teeth, in these cases bleaching of teeth under supervision of the dentist can nullify the effects. It is advisable to use the minimal concentration of bleaching gel, for the young adolescence. Young patients most likely will start with concentration of 10% carbomide peroxide, and evaluation by dentist is needed. Of course prior to starting the bleaching of teeth, the dentist must check for any leaking fillings, and cavities to be restored.Cosmetic Dentistry can be done in adolescence age !

I agree with the previous doctor and my question is why? An adolescent's teeth are usually very very white.

It is not recommended.  The nerve tissue within the tooth is much larger in a young person.  It is better to wait until they are in their 20's before using bleaching products to avoid possible nerve problems.  Good luck!