The DIN does have to do with height, weight AND boot sole length, so it is possible that his DIN has changed. Plus, you should recheck your DINs every season since you might have grown or gained/lost weight as well as changed boot size or skill level.
Yes, you should be fine fitting the new boots on the PX12s, as long as the binding is set towards the middle of the track or closer to the break as of right now. The PX series has one of the longest tracks of non-demo bindings.