Vocal registers refer to the use of the larynx when singing and in this case the range - basically he's losing the high notes. It happens to everyone with age, especially pro singers. He's using different techniques to reach higher notes these days but if they're relatively new to him he probably can't use them that often... think of it as vocal stamina.

Tbh for me Jon's always been about the showmanship anyway, he's worked out ways around the bits he can't sing anymore so it's luckily for him not such a big issue.