He was absolutely your emotional tampon and now that he's got somebody else to dote on, you're feeling threatened or possibly just feeling some false emotions that you think exist because your safety net is gone.
It's not to say that you don't like him, but if he were to come to you today and say, if you want to be together, I'm ready to make that move with you, would you be ready to commit to him? Methinks not.
Now it's possible you care more for him than you thought because maybe you were taking him for granted in the first place. Glory day. You do have a heart. But I'll bet that if you wait this out you'll realize that you liked him as a person but you liked what he did for you and how he made you feel more. It wasn't about him so much as what he did for you to fill whatever need you, well, needed.
I don't think there's any need to tell him how you feel unless you really have determined that you want to be with him, and even then be prepared to either a) have to be in a relationship, or b) get kicked to the curb for not respecting his new relationship. It's high risk, high reward.
Point is, you really need to think about if its him you like, or just what he did for you. Once you make that determination, grasshopper, you will be able to proceed.
I like the last sentence =)
If you had no feeling beyond friendship BEFORE he started dating this girl then he definitely was your emotional tampon and what you're feeling isn't real attraction. Keep it on a platonic level and don't make your problem dealing with this his problem.