All he needs to do is delete the site from his account and then you add it to yours .... if you have a good relationship with the previous webmaster ... if not... contact suppoert with your problem and the actually will help you ... may take quite some time, but eventually
It's simpler than that now since there can be multiple people authorized for one site. Just register for the site under your own account. Both of you can be authorized at the same time.
The previous guy is either authenticated via a meta tag or a file in the root directory that starts with 'google...' If you don't want him to be authorized, just delete his file or meta tag and the next time Google checks he'll loose his authorization.