Is he Neutered... if not lets think about getting that done, the smell is their natural sex attractant odor.. His behavior maybe nothing more than being bored. Since the vet says there's no UTI, I'd say ok, its a really easy thing to diagnose. A cat with a UTI will strain in the litter box, and try to go pee pee in other places too, straining to go. The vet and anyone that has experience with kitties knows that the bladder is located high up between the hind legs. UTI makes it fill with urine and can feel hard or firm, and is painful if you apply pressure up there.. I would also try to get him to drink more. Try a "Low Salt" or "Salt Free" chicken broth for cooking, found in any grocery store ..... he might like the flavor and you can dilute it with his normal water... By The Way, I only give my babies Filtered tap water. I use a PUR water filter, you can also use one of the pitcher types, PUR or Britta makes. that way you're sure that the water doesn't have too much chlorine or other impurities in it.... my 18 year old neutered male has NEVER had a UTI and drinks lots of water on a daily basis. I think his bladder & kidneys work better than mine ..