I had trouble triggering his scenes as a friend recently as a Qunari. They're basically identical except for the final cutscene.
You need to complete the following after you reach Skyhold, but before What Pride Has Wrought:
- Ask all of the questions from 'Tell me about yourself' and 'Tell me about the fade' dialogue trees.
- If approval is high enough, a special option in the top left will show up after you've talked to him enough, saying 'Tell me about yourself', which will trigger a cutscene in Haven. You need to pick the flirt dialogues in this cutscene to begin a romance.
- If approval is high enough, a personal quest will trigger after this, 'All New, All Faded For Her', which you complete in the Exalted Plains.
- After this, if approval is high enough, you will have a cutscene on the balcony in your quarters, at which point you can lock in (or break off) the romance.
You need to do this before What Pride Has Wrought aka Arbor Wilds/Temple of Mythal. Your final Solas romance cutscene will occur after you finish that quest.
Hope that helps 