Whose recipe is he using that has baking powder? Steam and the eggs are usually the rising agents in popovers. I use the recipe in the King Arthur Baking Companion:
3 eggs (sometimes I use 4)
1 1/2 cups milk (Usually 1%)
6 1/4 ounces AP flour
1/2 teaspoon salt
4 TB unsalted butter (though I find 3 is sufficient in the batter, I use the 4th TB to grease the heated pan.)
Best to let it hydrate for an hour or so.
Maybe his oven isn't hot enough, for the mini-popover pan I do 20 minutes at 400 and then 20-22 minutes at 300.
I haven't tried to keto-friendly this recipe yet, at 11+ carbs/popover (12 mini popovers per recipe) that's probably desirable. Like potato chips, it's hard to eat just one popover.