Bec,
My employer did this to me, you don't know what circumstances that may come up whilst on leave. I agree tell him nothing but that you have intentions of returning. My employer rang me constantly from the time Eliza was 10 weeks old wanting to know if I wanted to return earlier..they have no right, your right as a full timer is maternity leave and whether he has to pay someone more, that isn't your fault..even thou it sounds like he could advertise for a maternity leave position and train them before you leave.
Leave the door open, and tell them nothing except you are having 12 months mat leave, and if you don't want to return...write a letter of resignment just like me!!
Bec