Jon still has a pretty good range for the average singer.
As I said in an earlier post, the problem is that Jon uses too much chest voice instead of lightening it and adding a bit more head voice to it. Using too much chest voice for the higher notes can easily make you flat and even shouty.
And another reason being that his voice has lowered overall.

These two are the reasons why Jon sounds so shouty and is flat on the high notes in 2017/18.