I'm sorry, I don't believe for one minute that he was charged as a sex offender for nothing beyond kissing. There's either something you don't know, or something you're not telling us. Someone is lying here about what happend (beyond the girl lying about her age), and I have a feeling it's your angel of a son. No matter how honest you claim he is.
Whether you like it or not, the law is not going to hold the child responsible for lying about her age. Lying about her age is not a crime. Committing a sexual act of any kind upon a 14 year old is.
Since clearly the only answer you are willing to accept is that someone other than your son should be held accountable for his actions, please contact your elected representatives and tell them how wrong it is that a sex offender is required to register as one, and how a 14 year old should be held responsible for acts that she is not legally able to consent to. Let us know how that comes out for you.