There could be a number of reasons. He feet could hurt or there is some sort of irritant that he's trying to nurse. He could be itchy in that paw and can be cause by something he stepped in or his diet, allergies. He could be developing an obsession of the oral fixation sort and that could stem from boredom, anxiety, nervousness, etc. May he keeps stepping in food crumbs and is just trying to chew it off night after night, lol. I would take him to the vet to get him checked out.