If you have worked in the same role for 12 months as a casual you may be entitled to the 52 weeks. I would also argue that you didn't leave the company after the first lot of leave so you actually should have continuity of service from when you started minus the time you had off.

On the other hand if you didn't get anything in writing about returning to work early than originally intended then your boss may have a point. Although if you never had any timelines set in writing then he has no case and you can demonstrate you have been working regular hours for the 12 months then you can prove your service.