You have checked whether he's just spilling it, right?
You could use 28% as a supplement--maybe mix it with the 20%, or measure a certain amount and serve it as a mash so he'll eat that first and then he can fill up on the 20% feed afterward.
Or I suppose you could give him two feeders, one with 20% protein and one with 28%, and see how much he chooses to eat of each one
I found an article that mentions several studies about protein levels:
https://www.sciencedirect.com/science/article/pii/S0032579119381106
"Milne (1932) observed an adverse effect on the growth and liveability of chickens fed diets containing 38% protein. Heller and Penquite (1945) found poor growth in chickens on diets containing 35 or 40% protein. Almquist and Asmundson (1944), Heller and Penquite (1945), and Singsen (1949) obtained satisfactory growth of chickens fed diets up to 30% protein."
Based on that, 28% protein will probably not be harmful, even if it is his sole diet.
I wonder if he just needs extra food, because of a growth spurt or something? Or maybe he spent some amount of time being chased away from the feeder, before you put him in the cage? If that happened, he should settle down once he's gained the weight he needs.