I found an article about this recently, this is what he whispers:

"So Dad would take the Sundays off,
And that's the only time he could ever get any rest,
And so, because we were loud on Sundays,
He'd make us hold his construction boots over our head, till he'd sleep
And they were really heavy boots
And I'd used to say dad come on please
And like start crying, cause they re too heavy."