Formation of tartar on teeth is dependent on saliva chemistry and pH. A more
basic (higher pH) saliva tends to form tartar faster and in greater amounts
(increase in precipitate). So it is a factor of his body chemistry (genetics)
People with increased tartar formation are at higher risk for gum disease
(Periodontal disease). He may better benefit from 3 month maintenance visits.

Regarding Plaque formation: We all form Plaque on our teeth (dental plaque is
a biofilm made of bacteria and the organic matrix they produce to stick to
teeth) once removed by brushing it returns within 24 hours.  Plaque formation
rate is pretty constant among people. However thorough plaque removal is most
definitely not. Your son needs to learn to use his tooth brush better! If he is
not spending at least 2 minutes brushing 2-3 times a day he is definitely not
doing a thorough job. Electric brushes may help, but he still needs to spend
the time and place the brush where it belongs: at the gum tooth junction.
brushing only cleans 3/5 of the tooth. Flossing cleans the remaining 2/5. So floss,
and brush for 2 minutes 2-3 times per day and the plaque problem will be gone.

Regards,
Alex Shvartsman, DDS, FAGD