My dd was fully engaged at 37 weeks (and always really low before that, anyway) and only came 3 days early. We were always told in our antenatal classes that it means nothing, other that the baby is heading in the right direction!!!
Some of my friend's babies never engaged and they came around the right time, so I think that they just drop down whenever suits them!!! Lol!
Try not to worry, he will come when he's ready - although I know it feels like an eternity at this stage!!