If a Boy Scout wants to work with Cub Scouts, but his SPL, and SM, will not approve his being a Den Chief, he can still volunteer his time to help a den, he simply does not get POR credit for it.
My question would be, if a Scout is already doing all of that work on his own, and he is in need of a POR, why on earth would you NOT approve him as a Den Chief, and give him POR credit for it?
Does he want to work with Cub Scouts or just want to hang out with them? Big difference.