I led a guy on, now I feel terrible. Watch
I would also advise against remaining friends, even if he suggests it, because for a while all he'll want is to be back together and being around you will make it worse.
If the roles were reversed and you were a male leading a girl on you would be crucified.
Do you see yourself with this guy in the future? What if he becomes even more attached and starts wanting to go steady? It's going to be even harder then. You just have to man up and do what's right. Be honest with him.
And how are they not already 'going steady' in the context of the British 'dating' process?