I thought I did recruit him as an agent but maybe I didn't finish the quest if the marker isn't there.
For me; it worked like this:
Quest marker points to Dennet. He refuses.
Quest marker points to his children. They have several tasks that need doing, some of them large.
Quest markers point way to those tasks, some of which extend beyond the Hinterlands.
Quest markers point way back to Dennet's children when done.
Then the quest marker disappeared, but the quest still sat in in the journal.
So then I found Dennet and had to convince him to provide horses. I had three persuasion options with which to do so. One was related to a perk I'd taken, or I could have had Cass do it for me, or I could have had Dorian do it for me.