About 5,000 minimum. I mean look at it - he's got the balls to ask $1000/month for submissions and is even asking surfers for donations. Very smart - very brave. Regardless of what people think about "greediness", we're in this biz to EARN MONEY! I'm not going to rant but sometimes I feel like yelling in some people's ears, "WE'RE IN THIS BIZ TO MAKE MONEY YOU DUMB FUCKING MUTT!" But you can't send 10,000 and expect to get back 40,000 - he's clearly much smarter than that.
Remember that 5,000 clicks is pretty much small fish to him, 8-10,000 is more of a safe bet. Just mail and ask him for a link trade, but don't tell him how much traffic you can send. He'll mail back and ask you. Give him an exact figure, be prudent, don't lie.
It's best to do it this way because giving full details in the first mail sort of shows desperation. The fact that he's mailing back and mail is going back and forth allows you both to become familiarised with each other... the more mail flows, the more you'll trust each other. I don't like to be unsure, I always do things link trades this way.