Okay, disclaimer: I might sound like a jerk in some of this, but I'm only saying this, because you have to learn from these mistakes. We're talking money here
1) Don't do bussiness with 13 year olds
2) £15 is not steep, you could've asked for at least £20 for the stuff you'd be making for him
3) Like TakaM said, arrange the payment ahead
4) Your art is not good enough to do paid work, sorry
5) When someone tells you to follow a certain style, follow it, yours is completely out of style
6) You have no proof that he wouldn't have paid you if you'd have finished the job with the quality he expected
7) It would take me exactly 1 minute to remove those preview fonts, he could've ripped you off any time he wanted...
So, I'm sorry, but I'll have to pick sides for the other guy