Signs come with problems of interpretation. You have to look honestly at the
whole situation of why he might have approached you in particular.
But basically your problem is finding opportunity. Not knowing your office and its politics I can't know how difficult setting something up might be.
Have you any reason to approach him (work-wise)?
(While taking care not to be obviously watching him) does he do something habitual, like pop out for coffee or lunch, so you could time an interception to say hello? (I mean, catching him in the lift / elevator is a good one.)
Does he show signs of interest in anyone else in the office?
If you've known him a while you know his basic circumstances so you have some gauge of your chances. What's his position in your department?
Good luck.
♥