IMO it wasn't a bad idea per se considering that Dubois was desparate. Unlike the rest of the match, Usyk was squared up for the kill. Dubois slipped Usyk's 2, so Usyk was at full extension, so Dubois threw the counter. Unfortunately Usyk took a half-step back with the hook, slipped Dubois' haymaker, and cracked him.
He was smart enough to remember how much better Usyk is at boxing then he is. Too much intellect probably. He needed to forget that he'd already been outclassed by Usyk and go full ******.
I think he did go for the body, but Usyk took it away almost immediately with range control, his lead hand, and counters. When Dubois did get off to the body, Usyk blocked it. Seems to me that Dubois was going for the headbutt in Round 4, but Usyk took that away as well. Brilliant performance from Usyk.
Dubois is a solid fighter and can still beat many heavyweights. He was never going to beat Usyk without the aid of father time. He's too stationary and doesn't move his head. I do think he needs to get better defensively, he does get hit a lot.