Male cats are known to be prone to develop urinary stones, especially neutered ones. My first action would be to have the urine checked for crystals under a microscope. The vet needs a few drops of fresh urine that you can transport in a clean, airtight plastic or glass container. Maybe he does have a stone which irritates his urinary tract and the medication relaxed him enough for the symptoms to improve. But if there is a stone, it won't go away with some relaxing pills.
If you don't trust your current vet, go to a different one and describe to them the history of your cats treatment.
If there's no stone or crystals in the urine, You need to actively reduce stress for your cat. One way to do so (especially if the cats don't always get along) is to separate them. Keep the doors closed for a week and see how they react. They might need more entertainment from games now because they cannot walk around as much.
Providing shelves and other furniture where the cat can walk around above ground can also reduce stress. You could search the internet for "cat shelves" or "cat walkways" for inspiration. Even if the cats don't get along, if they can avoid each other vertically, you can avoid stress.
And please keep in mind that the simple act of sharing a litter box can be stressful as well. The rule of thumb is to have one litter box for each cat plus one extra, if the space allows. And the litter box should be cleaned (clumps removed) as often as is practical. Don't let it lay in there all day, you wouldn't want to use a dirty toilet either.
You could also try switching to grain-free, low mineral wet foot. That has the best composition to prevent urinary stones in a natural way. There is also special urinary cat food, but some are cheap and some are very expensive. And there are 2 different basic types of urinary stones (based on their chemical composition), so not every "urinary" food prevents all urinary stones. Read more about preventing urinary stones here.
Switching cat foods should be done over a period of 3 - 5 days to prevent uncomfortable side effects like bloating. On the first day, mix only a little bit of the new food with the old one. Then increase the amount of new food every day until you only feed the new food. You should feed the new diet for at least 4 weeks before you can expect to see any effects.