That's the great mistery of the show. Rebecca knows, because of that project they both worked on. Maybe Evan and Casey know too, since they were both really close to Cappie but other than that nobody (except the writers, of course) knows his name. Even the teachers call him Mr. Cappie!

Apparently Scott Michael foster asked the original writer what the name was (yeah, even scott doesn't know) and he said, I am not going to tell anyone. I've wanted to know since like the first ep.

Rebecca knows but they never said his real name in the show. maybe in the future they will. even the cast doesnt know at all.

Nobody that watches knows. Rebecca knows, and possibly Casey and Evan know. But it has not been revealed yet. Rumor has it that we will find out on the series finally or when Cappie graduates.