I always got it to trigger, when I asked him about courtly dances, when he tells about his life as the son of a nobleman and then say something nice at the party to him - that I wished, we could have danced or that I would miss him at the party, basically hints, that there is interest. I like, that it is pretty subtle in Wylls romance. I think important is, that you talk to him at the party and show interest in him. Then when you go to bed, select thinking of Wyll ( something along the line).
I think apart from Lae'zel, Halsin and Astarion, who all ask you, you have to start a romance now. Even Karlach: I had her scene popping up randomly, when the approval was high enough before and now, I have to tell her that I like her at the party.