There is no guarantee that his rate will decrease, but on average, rates decrease as drivers get older. Calling his agent might help, but only if the agent works with multiple insurance companies since his current company is unlikely to make adjustments to the rate in the middle of the policy term. I would recommend that your son shop around
with as many companies as possible to get the best rate. If he has been insured for 6 months or longer then he should be eligible for lower rates because of the discount associated with maintaining coverage and also because of his age.