To the best of my knowledge (minimal), if the code contains his copyright notice, it belongs to him unless they can prove otherwise through documented transfer of ownership or license.

I once got into a bother because 3 months into a contract for a very large computer company, someone noticed that every file of code I had produced had a standard header comment containing my copyright notice. I defused a potentially nasty rucus by explaining that it was produced by my editor when I created a new file, and that I simply hadn't thought about removing it. I offered to remove them all immediately. They still made me do it in front of a lawyer, and sign-off in triplicate each and every one of the files.

The upside was that it caused them to review my contract--a standard boiler-plate that I had signed when I started, and in the process I voiced my disquiet at their interpretation of the all-encompassing "We own all code you write whilst under contract to us". The basis of my argument was that as I was employed by-the-hour, I was not under contract to them outside of those hours. It got escalated up the food chain several times, but the net result was that my interpretation was deemed the correct one. (In the UK. Not sure about elsewhere.). My contract, and their standard contract were re-written to state that explicitly.