try making him follow you and then talk to him?
there was a similar thread about this a while back...could try a search, and read through some of the posts to see if anyone else had the same issue..
setstage ms04 700
go to him with another companion, tell him to follow, you'll have the dialogue option then. after the dialogue, take your original follower back and, when prompted where to send hancock, tab out of the selection list, that way he'll stay in goodneighbour (which perfectly makes sense in his case since, other than the other companions, he can die and hence is best kept in a safe place. for his own safety of course.
edit: problems like this, or preston inevitably stealing your settlements so you can proceed with some entirely different quest, etc, would totally NOT EXIST without that darn dialoge cross thingie...
In order for it to work for me, I had to get him as a follower then dismiss him, then talk to him as the shroud...