The best approach to requesting leniency, assuming the offense in question is the result of atypical behavior, is to point out your son's clean history and positive character attributes. It is also a good idea to point out how your son has taken steps to better himself since the crime.
Certain aspects of sentencing are beyond a judge's control. Crimes at the felony level often have mandatory sentencing guidelines that the judge must remain within, whether or not he desires to be lenient. So the severity and nature of the crime play big roles in sentencing. Also, your son's past is important. If the offense in question is his first and he has been a high achiever involved in the community in the past, a judge is more likely to grant leniency than if your son has past offenses and has been struggling to strive within the community for many years. If your son is young, age may persuade a judge to minimize sentencing, although it may have no bearing at all. Beyond pointing out the positive traits and clean history of your son, make sure your letter is respectful of the judge and the decision he must make as well stress your regret over the crime and assuredness that a lesser sentence is likely to aid your son in his reform.