It's easy to just say, "Tell him he can put on a 1Bman's mitt or play DH, or both. You have no choice."
In reality, I'm not sure how much of an issue this would create. I don't see the "happy-go-lucky" Devers as a pouter, but I'm also not sure I'd want to try and find out.
We are a much better team with Bregman at 3B, and in some ways, it smooths the path for Campbell at 2B rather than the OF, of gasp.... SS.
I'd try hard to explain to him that he might think he's good on D, but 29 other 3Bman are better, including Bregman, who is top 5. Maybe don't tell him he sucks on D, but that Bregman is one of the best and is a longtime proven vet, who deserves to play his natural position, too.
If he remains adamant, maybe we need to ask what he'd do, if we force him to 1B or DH. How he responds might matter.