3 yr old Male cat won't stop spraying/peeing

So, a few months ago, my 3 year old (neutered) Male cat began spraying. When I took him to the vet, they found crystals in his bladder and after a month of taking medication and return check ups, they passed and he was fine. But recently, he's been spraying on the walls of our living room and I haven't the slightly clue as to why. There's aren't any other cats roaming the neighbor (most went missing or were eaten by wild animals), he knows where and how to use his litter box (he'll poop in there but that's about it), when we thought it was maybe something stress related, I took him back to the vet and we put him on some medication to reduce his anxiety (it helped for a week and then he want back to spraying), Feliway doesn't do squat and we've tried taking him outside more (because he'll meow like crazy to go explore outside) and playing with him more. My family is already looking into shelters to take him to, but I've become desperate in finding a solution. Please help!

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