A 15 minute attention span is perfectly normal for his age.
In fact, 15 minutes is pretty good.
Dr. Roy Benaroch, a general pediatrician, wrote this on his blog regarding toddler attention spans:
Normal toddlers can have a very short attention span. At times, they’ll zoom from toy to toy like a hummingbird, barely touching one thing before moving on to another. It’s common for toddlers to lose interest three pages into a story, and completely lose interest in a new toy by the time Mom gets the package open!
Occasionally, toddlers will spend a solid five minutes or more on one toy, but expect that to be the exception, not the rule.
The Children's Hospital of Richmond at VCU agrees, saying that the length of the attention span can also vary depending on the subject, environment, etc.:
Realistically, children, ages 15-30 months, attend for 10-15 minutes when highly motivated. The complexity of the task also matters as a demanding task may require so much concentration that attention cannot likely continue for more than 5-7 minutes.
That same page indicates that 15 minutes isn't bad even for a pre-schooler:
For preschoolers, attention span will vary by interests and situations. For example, when exploring in a familiar place with a new toy, a child may attend for 15 minutes or more. However, in a distracting place or when others stressors are interfering (illness, hunger, thirst, need to use the toilet), a child may appear distractible or less attentive.
As far as when you want to start worrying about a child's attention span, earlyinterventionsupport.com lists some milestones, broken down by age:
Ages 8 months - 15 months
Any new activity or event will distract your child, but they can usually attend for one minute or a little longer to a single toy or activity.
Ages 16 months - 19 months
Your child might be restless, but is able to sustain attention to one structured activity for 2-3 minutes. Your child might not be able to tolerate verbal or visual interference.
Ages 20 month - 24 months
Your child is still easily distracted by sounds, but can stay attentive to an activity either with or without an adult for 3-6 minutes.
Age 25 - 36 months
Your child can generally pay attention to a toy or other activity for 5-8 minutes. In addition, he/she can shift attention from an adult speaking to him/her and then back to what he/she was doing if he/she is prompted to focus her attention.
Ages 3 - 4 years
Your child can usually attend to an activity for 8-10 minutes, and then alternate his/her total attention between the adult talking to him/her and the activity he/she is doing independently.
