Urinating outside of box.

Hi there,

I have two cats, a male (Duke, long haired tabby) and female (Bella, Blue Point) both 4, brought home as kittens at around the same time. They have both been spayed and neutered. When they were brought home, litter training them was a breeze. For the past couple years Duke has found a love for the outdoors but only the past couple months has he also found a love for urinating outside his litter box, usually on curtains, sofas, speakers and other mainly absorbent but obvious objects around the house. When I let him outside, I usually sit in the kitchen and watch him as I live in the city and letting him out alone is not something I think is safe and too keen on. He's restricted to the backyard. Mind you, when I do let him out it's usually when he wants to and for the most part he either comes in on his own or when I need to leave. When he's brought in involuntarily, that's when he almost always urinates. Now as I'm a student I don't have money to be putting towards vet bills as I'm almost positive this is a behavioural issue so I've been trying some things on my own such as getting another litter box (thinking he didn't like to share), changing the litter daily, cleaning the litter almost right after its used, spraying the furniture with cat repellent, working out my schedule so he can stay outside longer, etc. But still, as soon as it's time for him to come in and my back is turned, the smell of urine is lingering and it's a hunt to find it. Now getting rid of him is not an option but this issue has to be solved.

Any help would be ever so greatly appreciated.

Thank you,
Tasia