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).
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.