According to this site:
there is 45mm of adjustment. That sounds like a lot, but it may not be for your specific binding.
To know for sure what your binding will do (I never trust published specs), slide the binding in the track all the way forward to its 'shortest' working position, mark the location, then move the binding all the way back to its 'longest' working position. Calculate the difference. This is the bindings range of adjustment.
Next determine the range of adjustment that is required to handle the largest and smallest boot. For the nephew, ask him to find the 3 digit number followed by mm or MM on his boot. That is his BSL. Don't let him confuse Mondo size for BSL. If the BSL range is less than the bindings range of adjustment, you'll be fine. Just set the jig to the size of the shortest BSL plus half of the BSL range. Since the jig will place the heel piece dead center in the track for the selected BSL, you'll have plenty of room to move forward and back.
The other consideration is the toe location. Boots shorter than the BSL that the jig is set for will be slightly forward of center and those longer will be slightly back of center. Since the range between 27 and 28.5 is likely only 15mm, then this will be hardly noticeable (IMHO) for those using the skis. Since your boots are in the middle of the range, you'll be pretty close to center.