Everyone likes hearing that they are cared about. If you are already dating this guy, you know it must be because he likes you as well. This offers you a bit of safety - you're not taking a huge risk and telling a non-boyfriend you have feelings, after all.
Talking is a classic method. If this makes you nervous, try texting, chatting online, emailing, or writing him a letter. More creative ways include writing it on a cake, making a homemade card, or really anything you can think of.
The most important thing is to express it, somehow. Once he hears how you feel, it'll hopefully open up some really good chances for you guys to communicate about it.
I wish you the best of luck!