If you have plenty of rabbits to ferret i suggest you buy a collie blooded dog for the boy. They have the intelligence to pick the game up very quick.
Maybe a dozen outings with a dog that allready marks just to see how your new dog marks as there's a massive variation in how ferret dogs mark, from very subtle signs to 1000% unmistakable mark lol.
As has allready been said it will probably be difficult to drop on a ready trained dog. So I suggest buying a young dog or pup in yourself.