Send the singer a mix with the bass, drums and guitar and ask him to sing to the beat.
It may be that the horn riffs and piano stabs put him off or he hears crooning as across the beat but needs to swing with it.
In addition you can cut up the vocal into phrases and slide them about to sit better.
Nice work - he's a bit essy as well.

Last edited by rayc; 10/17/19 11:01 PM.
Cheers
rayc
"What's so funny about peace, love & understanding?" - N.Lowe