Though inderstandable, you needn't feel guilty about wanting full attention and support for your therapy. That said, I think a lot depends upon the relationship you have, and how you feel about this knowledge. I would engage normally, and then if you feel he's "off" in your session, bring that sense up. No direct reference to FB and what you know. Not because I think you should hide it, or feel badly about it, but because I think it's respectful to allow him the prerogative of whether he shares this news with you or not. I would do that with anyone I knew was grieving. Then take it from there.