With urinary crystals, flushing them out before they can form to a problematic size is key; struvite crystals are primarily caused by insufficient hydration and alkaline urine. The first step is to move away from kibble, grain-free or otherwise, and switch to a canned or raw diet (depending on what your cat will eat, some will not eat raw foods). Cats evolved to get most of their hydration from food, not standing or flowing water sources; canned cat food is ~75% water, where kibble is ~5-10%. Note that this doesn't mean you can skip out on offering standing/flowing water options; ideally, provide both bowls and one or more fountains for your cat, so he has choices on which to drink from.
Grain free will be the next consideration when choosing the new food options; most higher-end foods will be grain free, but as always, read and understand the labeling before making a choice. Higher meat content foods will result in a more acidic urine, helping to prevent crystal formation; the plant matter (cereal grains, etc) in the prescription diet skew the urine pH toward alkalinity.
If your cat is particularly prone to crystal formation, you may also wish to look at lower-phosphate foods, however, this information can be difficult to find from most manufacturers, and is rarely on the labels themselves. You'll need to look up charts compiled by the company, which may not be available.
While in most cases, it's worth understanding and keeping your cat on a vet-prescribed diet, unfortunately, in the case of Royal Canin Urinary SO, the "secret ingredient" is extra salt to attempt to provoke the cat to drink more water; it's much easier to introduce hydration to the cat via food than the feline equivalent of free bar snacks. Do speak to your vet about the change in food, anticipate some pushback, and agree to additional monitoring the cat if needed during the dietary change. I've been in your situation in the past, with the same prescription diet, and found definitive positive results in my cat's health by moving him from SO to a grain-free, meat-based, canned diet; the results were proven out by subsequent urinalysis performed by a skeptical veterinarian.
