He can't do that. In fact, it's against eBay's rules for him to hold onto your item until the payment is cleared - usually, they have access to the money when they provide a tracking number, but of course, he can't do that 'cause you're in Mexico. Yes, it sucks for sellers, but that's how it is. Furthermore, him saying "I won't ship this until you leave a positive" is feedback extortion, as defined by eBay's rules.
He can't ask this of you. The only thing I can suggest is that you go to eBay's forums and ask for help, but I would report his ass ASAP. :P