I've tried looking this up, but didn't find an answer. you can drop hints sometimes while talking to him, but the game makes it extremely obvious when you can go full romance, end of act 3. The OP asked for inter party romance if I recall. Red prince also gets some action at the end of act 2 when you reach the caravan. For best results I suggest using the red prince. There is romance in act 2 if you talk to the waiter about some stew.
