How can I stop him from reblogging posts from my blog?
The answer is "you can't".
Is there a setting allowing me to approve/disapprove reblogging posts from my blog?
No there is no such setting.
The ToS here allows WordPress.com to publicize our blogs to increase traffic to them and to this platform. When it comes down to it the more page views the more income making opportunities there are for WordPress.com. That's probably why the reblogging feature idea was hijacked from Tumblr and implemented here ie. to increase page views to the site, though I'm sure that Staff will provide baffelegab to the effect that they do it all for us.
There is no ability for us WordPress.com bloggers to be selective with regard to who reblogs our posts. If another WordPress.com blogger has a blog that we would never even wish to be associated with our blog they can reblog with ease ie. we are hooped. We are left with posting a comment to the reblog site we do not wish to be associated with and begging them to take our content down by deleting the reblog. The only other solution with regard to this and many other features that some consider to be undesirable is to blog off ie. to move our content elsewhere.