If urine sits in a glass container for a extended period of time crystals will form. The urine itself has all the ingredients in it to form crystals all by itself.
This is not a problem unique to your dog. It is a very common problem, and since it is highly treatable there is no reason why your dog will not continue to live a long and happy life with treatment and prescription food for the condition.
If the crystals are small than a prescription diet should be able to eliminate the crystals by altering the pH and eliminating the chemicals that will contribute to future crystal formation. Only the correct prescription diet can do this. Since there are several different kinds of crystals, the diet must be prescribed for the type of crystal that your dog has. You did not identify which kind of crystal your dog had, but I am sure that your vet will prescribe the correct formula.
Only a prescription diet and time will eliminate the crystals. If the crystals are too large to safely pass through the urethra they may have to be eliminated by surgery.
Cranberry tablets or capsules are effective because they prevent adherence of bacteria to the lining of the bladder wall. They will help as an adjunct to other therapies to eliminate urinary tract infections and to help prevent as a preventative, but it will not eliminate crystals. So please give your dog cranberry.
Glucosamine is not only just for joint health. It can help the lining of the bladder to aid the bladder's lubrication properties and also help to prevent bladder infections and subsequent crystal formation. Please place your dog on a glucosamine, chondroitin supplement, if he is not already on one.
Increasing your dog's liquid intake will help to flush out the crystals more quickly, this process can occasionally be supplemented by Sub-Cutaneous fluid therapy, if urethral blockage is not a concern. Using an all canned prescription diet will also help increase liquids.
Allowing your dog to take frequent urinary bathroom breaks will also help to flush out his bladder and will prevent the urine sitting in the bladder long enough to cause crystal formation. Increasing his liquid intake will aid with this.
You may be interested in an herbal formula that helps prevent urinary tract infections since crystals are rarely found without a concurrent urinary tract infection. The herbal formula is called Eight Righteous and is inexpensive, safe and is widely available online, including at amazon.com
Lastly, since your dog is a mobility service dog you will have to contact the mobility service dog organization that he is connected to and find out if they will be able to help with your bills and the cost of prescription foods.