I can only associate his pronunciation and rhythm stuff to him being emotional and/or confidence stuff (I am sure it's not fully gone).
The thing is, if he talks and pronounces everything correct he should be able to sing and pronounce correct.
He just needs to try pronounce correct somehow, that is the solution instead of ignoring pronunciation, vocal cords have almost nothing to do with pronunciation except making the sound itself (Say AAAAA and you hear it) and the pitch.

He barely moved on stage due to lack of confidence 2013-2017. Due to lack of confidence he never did any sort of actual improvisations with vocals or so, not even really rasp in voice, etc at all. When he's very confident and/or happy he could/can do all of these in a single concert (Australia 2010. Sydney. Happy Now, STBI and SIBSN are good examples from that concert.)

In Sao Paulo, etc he regained confidence and started pronouncing words better due to that, etc.
He needs to fully regain confidence in order to pronounce words better.