Rat Terriers are famous for their boundless energy and enthusiasm for chewing things up, especially when they are young like yours. I suggest
kennel-training. Buy a
kennel big enough for a bed, food and water, plus enough room to play with a half a dozen toys you will add. The toys should be various types: a rubber ball (big enough not to get caught in his mouth), a large rawhide, a stuffed animal he can chew up, a rope-toy, and maybe an old sneaker with laces and eyelets removed.