He knows something that you want to know, but he has been sworn to secrecy. It is possible to get someone to share little bits of this and that without resorting to illegal, underhanded or desperate methods.
Start chatting him up regularly. Don't talk about anything in particular, just be amiable and friendly. Get him used to visiting with you.
Step2
Listen carefully when he speaks and ask appropriate questions. Show that you are interested in him. Stay away from the secret subject for the moment as you don't want to scare him off.
Step3
Begin to share with him little tidbits of information. Respond to his questions in a serious way to demonstrate your thoughtful, serious nature. Every now and again, hesitate in a way that invites him to pump you for more.
Step4
Begin a program of disinformation and feed him information that is just a bit wrong and let him correct you.
Step5
Now you can start asking discreet follow-up questions related to the secret subject in an innocent manner that suggests to him that you will be okay with the information.
Tips & Warnings
Do not attempt to access personally identifiable information about another person; it is unethical and possibly illegal. Do not attempt to access secret information from the government or military.