I wouldn't mention it to the boss until closer to the time, ie. wait until things start to get cemented into place, especially if your boss is just "talking" about it at the moment. It IS possible - but there could be unforeseen circumstances (like for anyone) so I say if your pg has been good so far, go along with it until closer to the dates, see if it can't be brought forward (a few weeks makes all the differences) and check with your doctor/midwife/obst. before you go.
All this is especially neccessary if you want to return to work and keep good relationships with the boss - you shouldn't have to, but it can be a fact of life.