If you have it in writing (and it's clear) you can fork the software at the last commit. Be aware that you risk being fired. OTOH they are risking a wrongful termination claim / lawsuit. Option 3 is to negotiate back pay for the work you did based on your best estimate of the cost.
There's also some risk to your boss -if he's given you permission in writing to release the code under an open source license, but he did not have the internal authority to do that - it's not impossible that'd risk him getting fired (instead of or as well as you).