My suggestion - take off all the bed sheets or bed covers. She might start to hide them under the covers and you don't want to sit on them accidentally. And you can keep the same box, in case this guy is so greedy that he doesn't let his momma go when she leaves the box. If the side will be too high, the kitten may not be able to get back, or she may not be able to take him back, though that sounds silly, if she wanted to, she would take him back. If she's still nursing him on the floor... maybe that's her way to feed everyone the same?