last year my pup (also terrier

) , like yours fed with Canidae, had crystals in urin and 2 bladder stones.
my vet reccomended S/D from hills, but it is even crapier than SO from Royal Canin, so i switched to royal canin for 3 weeks until stones and crystals melted.
Vet said he should eat that diet for 6 months, but i couldnt stand feeding this crap to my pup.
As i have heard from several people that Canidae didnt do well for their pups, and it didnt work for my pup either, i started to feed him Arden grange for pups until he was 11 months old. he was then put back on Canidae and Canidae is now doing great for him for the last 8 months.
try and put him on different low protein, low ash, low calcium and low phosphorous PUPPY food, and supplement vitamin C, this will lover pH in his urin and help to prevent forming of new crystals and melt those which already exist. there is one good vit C supplement "supet vita vet C" from Vetnil. But this is just one, i am sure there are plenty of different supplements.
bladder stones and crystals are common thing with young pups of small breed, but they rarely become that large that they cause problems.