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I'm sure many people are going to jump on here and say that this was a premature callout thread, and you should give him more time. I disagree. Three weeks is ample, and he has stopped replying. Hopefully you sent the payment NOT as a gift through PayPal. I would recommend waiting two days now that you have posted this thread, and then opening a PayPal claim/dispute.
You should also be able to get his contact information through PayPal. It should tell you his name, and probably his address. It is fairly easy to find someone's phone number. Give him a call, let him know why you're upset (as if he won't know), and mention that you really can't continue to wait.