It would be impossible to get him to drink as much cranberry juice as he would need to see a benefit. A powdered cranberry supplement is much more practical.
Studies indicate sixteen ounces of cranberry juice need to be consumed to obtain the same dosage as a single concentrated cranberry pill. Cranberry supplements and their use to treat urinary tract infections - by CD Gozzi - Helium
I prefer supplements specifically for dogs so I know the dosage is correct. Cranberry Relief is excellent.
Amazon.com: NaturVet Cranberry Relief 50 grams:... Cranberry Relief: Kidney Medication for Dogs & Cats - 1800PetMeds Cranberry Relief