Keep the cat litter box in a room he can't get to. Gate the door so the cats can get in and he can't. I know it's gross, but cat Poop is like a treat to a dog. I've read where no dog can resist it. And before I moved my litter box, I found Petey getting his own snacks when I wasn't looking
The cats will want a room they can go and get away from the dog anyway.
He is probably young enough you won't have too much trouble for him adjusting. Just be dilligent in NOT letting him chase and bark after the cats. Once he see's them run for their lives, it's a fun game
Both of my cats are declawed and fixed. They are both bigger then Petey too. Almost double the weight. He won't chase them unless my mom's dogs are with him for "Back-Up"
Kim