No sway back, and he's got long looking legs, I can't put him on 5.5
My initial thought at first glance was 3.5
Maybe I'm wrong. He's got coke can mass and where his neck connects to his body is wide like we usually see in older bucks. He definitely doesn't have a skinny looking neck.
One thing about this buck I would never pass him and when he's down I'd definitely pull the lower jaw and figure it out.
Good luck hope you get him!