His life and freedom would constantly be in danger. He could, without any help, discover and reveal highly classified state secrets, expose corruption, and find criminals anywhere--or wreak havoc on the intelligence community, endanger lives, and promote terrorism and hysteria.
He would be an extremely valuable intelligence asset to anyone who controls, tortures, or imprisons him. He would threaten anyone who was criminal or corrupt. EDIT: He could threaten anyone, corrupt or not.
He would probably ask himself this question all the time: "If I help this person, will it put my life in danger?"
He would be likely to guard his ability very carefully so as not to be discovered unless he wanted to be.
(edited) Depending on how his "gift"/curse works, it might be difficult for hime to say "As far as I know..." -- it would be up to the OP. Joe might say "I believe XYZ". Depending on his personality, would he leave himself in doubt about his answers? Whenever he has asked a question to himself in the past (which is likely often), he wouldn't be able to evade questions with "As far as I know." From a writing perspective, it would be difficult to imagine all the circumstances he might have asked himself some questions--it might be easier to avoid plot holes if he finds other ways of avoiding questions.
He might evade questions rather than provide conditionals, because if he made a mistake and suddenly went mute, he might give away information he hadn't intended. Even simple questions like "How are you?" could be met with something overly specific and useless, like "I'm here. How about you?" or with a question "Thanks for asking--I am trying to solve a problem--do you think you might help?" In awkward situations, he would probably avoid the topic altogether, by telling himself how beforehand. He would prepare. "I have a good chance of avoiding a tough relationship question by saying...".
If someone asks him a question point blank, he might say "That's a good question. Do you think so-and-so might have an answer?" (side-stepping whether or not he knows at all). He would likely get very good at this, so the OP might benefit by studying how people avoid questions and guide conversations.
Another possibility would be that he might mask his truth telling by making it appear he has some speech impediment. He would intentionally try to tell lies from time to time, just to make his speech seem more halting than it is, or do other things to cover it up.
He would know when he was going to die, and how. He only has to play twenty questions with himself a few times.
He would probably ask himself how to keep his gift a secret. Probably the best way would be to withdraw from society.
He might also hide his ability by working with someone else who can lie in his behalf--never answering a question directly.
He would ask of everyone he met, "Can I trust XYZ to keep my gift a secret?" and "Is this person someone I can trust?".
That said, he would likely end up in a network of deeply trusted friends who would protect him and his secret, while safely and secretly using the information he can get to save lives.
He would probably be corrupted by his own ability. The opportunities to get gain would be extremely tempting. He could play the stock market, among other things. He could just as easily become an evil supervillian corrupted by his own ability as a superhero.
He should not be a lawyer or in the legal profession, because that profession requires transparency and would expose his gift, putting him at risk. More likely, he would work indirectly for an intelligence agency which has guaranteed his safety and anonymity, and can be trusted to do so. Likely, he would work off-the-record, through other intelligence agents. He would try to be sure most of the CIA/FBI/whatever wouldn't even know he exists, and work hard to keep his existence out of any documents or records.
If he went public with his gift, most ordinary people would regard him as a super intelligent man who was somehow deluded or crazy. Criminals, governments, and others would look into him and regard him as a threat, especially if he started proving useful.
