You can certainly get a letter that indicates a successful completion of probation, although I have never seen one that indicates past drug test results unless it was to support a VOP. Ask for it in writing, addressed to the vicinage probation supervisor. Send it by regular and certified mail. Do not make it a complaining letter or try to show that the probation officer has put you off. Make it a NICE request, and cc the probation officer. Explain why you need it and point out the benefit you got from being on probation and how you want to accept responsibility for, and be involved with, your son.