Unfortunately, s/d is not the most palatable diet from Hills. However, it is important that you stick with the prescription food - it is a prescription and contrary to what most people on here think, if there is proof (ie. radiographs) then your doctor is doing what is best for the pet not looking for some kickback (Hills doesn't having a kick back for putting an animal on a prescription diet, or any real kick backs that I know about).
You probably do need to slowly introduct the food and not just give the s/d. Most likely, you can not add anything after you've made the switch (ie the gravy someone else mentioned) because the food works by altering the pH of your dog's urine to desolve the crystals. Adding other things to the diet unfortunately can make it so the food isn't working at all and surgery will become your only option. Make the transition over a week and if you are still having problems, here are a few options.
1. Ask your vet about another diet. Hills is only one of three (I don't count the Iams/Eukanuba brand since not many vets carry these prescription diets) prescription diets. Purina and Royal Canin both make urinary stone disolving diets. I, personally, see a lot of success from Royal Canin's Urinary SO. Your dog may accept one of these better than s/d.
2. There is a homemade version of s/d. You'll have to ask your veterinarian (or email me if he/she doesn't have it and you have to choose this route) for the recipe, but there is a home made diet version of s/d that your dog may take better. The recipe is made by Hills to mirror the action of s/d. They have a lot of home cooking recipes for thier prescription diets which is very useful for a picky eating dog.
3. Go to surgery. Your vet didn't just look at your dog and say, hey, I'll make some money and sell this food because I can. Your vet found a legitamate problem and has given you your options: a prescription food or surgery. I'll admit some vets are all about making money off the client, but this is the same path we offer our clients so its not like its uncommon. (This is pretty much written towards all these people on here who think the vet is the devil)
I hope you can get your dog to eat the food and that it works for you. I've seen some remarkable results with using a stone desolving food so it is definately worth the effort so keep trying. BTW, in case someone suggests this, there is no kibble form of s/d, only canned. Good luck!
PS. remember to measure the number of weeks the dr. wanted your dog on the food starting when your dog is completely on the diet, not including the days you transition. also, treats are a big no no on these prescription diets because they can counteract the food! so you'll need to get him on the diet and nothing else.