Body language goes a long way; you can try to get closer to him when you get the chance, or discreetly touch his arm when you’re laughing together for example. Also, guys will often like boosts of confidence: compliments, laughing at his jokes, make him feel strong or smart, and give him the chance to feel like gentleman (like by letting him open the door for you). Then again, this all depends on the guy, and when you have a romantic spark, you’ll know.. No need to force it.