Hi Tom,
I think you have to wait until after medicals and reference processing before they'll give you a date... that said, at my stage 3 one of the HR guys told us we would probably be put on the June course if we got through. Will also depend on whether you want area or aerodrome - there are no aerodrome courses until June (I think) and apparently that is already full up because the Jan aero course was recently cancelled.
If I were you I'd just be honest with your boss; let them know its coming but that you don't know the exact dates yet.