Everything depends on what was written in his contract. If in it didn't written anything about bug fixes, after end of contract he/she didn’t carry any responsibilities about bugs, which appears. You must be enough polite with him/her and negotiate about reasonable rate. If rate is very high you must make your trade off - to pay sum, or to lose unlimited hours to fix bugs, or to do nothing.
Venelina Jordanova
Outsourcing IT Services Ltd.