First of all, it is hard to find someone who has never dated anyone else or liked someone else. So forget about being his "second choice". Maybe you weren't. He wasn't ready for you. She wasn't his first choice. She was the easy choice. Don't hold that against him if you really like him. This kind of situation can work out wonderfully for you two.
My case: I had a girlfriend for a year and half who broke up with her boyfriend so that she could get to know me better. We were together for a year and a half. She is now engaged to that old boyfriend. They showing every sign of a happy ending story. He accepted not being "mr right now" and he ended up getting everything he wanted. you have to swallow some pride i guess, but there is alot to gain from that.
Sniff first, then scratch.