I think, for the most part, his changed vocal tone is due to his singing technique.

It seems like he's afraid to project the notes properly - maybe because he's afraid of going off-key or his voice cracking - so he's placing the sound at the back of his throat rather than projecting it forward.

Based on what he's said about having self-doubts, I think the way his voice started deteriorating physically in 2013 made him lose his confidence in himself and his vocal abilities (and it made the little flaws in his singing technique that had already been there since the 2000s come out much stronger), so now he's trying to play it safe - except he's just making it harder for himself with that singing technique he's using. Also the fact that he writes a lot more in his mid-range nowadays really isn't helping, because that's the weakest part of the human singing voice, and neither is the fact that he doesn't pronounce the words properly. He also always seems very tense while he's singing, which just fuels the other issues even more.

Obviously physically his voice isn't what it used to be anymore either, but I think if he changed his technique - particularly the way he projects the sound and pronounces the lyrics - his vocal tone would probably be a lot more like what he sounded like in around 2015 than what he sounds like now.