We have 9 cats. Our senior guy, who just died at 19 years old was a mane coon, and for his entire life he was obsessed with digging in the litter box for anywhere from 15 minutes to a half an hour. He'd dig in it, around it, and back in it until he was satisfied that he had sufficiently covered up "his business". Interestingly enough, after all that effort, often he completely missed the "business" and had covered up everything else but that! LOL This simply inspired him to try harder - and continue to miss!
So your kitty's behavior is normal, I believe. He is just trying to be fastidious.