Why is he suddenly peeing and pooping everywhere?

I'm trying to figure out why my roommate's cat is suddenly not using the litterbox.  We have a large, three bedroom apartment, which I moved into a couple of years ago.  I also have a cat, it took a month or so, but the cats are now friends.  We got a new third roommate a year ago, but nothing else has changed.  There is a large litterbox in the laundry room.  We also have a walled in deck room behind the kitchen and there is a cat door for them to go into the deck room.  We have two more litter boxes out there. We have tried very hard to keep them all pretty clean.  We all love cats and they get lots of attention from the three of us. 

My roommate, who owns the problem cat, is frequently not home and at her partner's place, which has always been the case.  When I moved in, she told me her cat had been known to pee on the sofas.  In the first year and a half, I lived with her, he never peed on anything, I was like, maybe it's a thing of the past?  Then several months ago, it all changed.  He started pooping on the back stairs that go to a back door from our deck room.  He would also sometimes poop in the house, on the floor or one of the sofas.  We started cleaning the boxes more often, but he would still poop on the outdoor stairs sometimes, even when the boxes were totally clean.

Now he has started peeing again.  Last night, I was sitting on our smaller sofa, talking to my other roommate, who was standing.  The cat jumped up on the large sofa, looked directly at me and peed on the sofa!  We know he is trying to tell us something and are worried he may be ill.  He has also become very thin over the past several months and he used to be very large.  He otherwise acts normal, likes to eat, drinks water, likes to play, killed a mouse not too long ago, etc. 

His owner was not home last night but I contacted her and she says she will try to get him into the vet today, which I'm hoping will happen.  She is not always the most attentive cat owner.  We try to help out too and give him attention. 

Anyway, I'm wondering if anyone has any ideas what could be wrong?

A trip to the vet is the first thing on the agenda, since he has lost weight and is also obviously trying to get your attention.  The vet should do blood work and check his kidneys, maybe also a urinalysis to see if he has any bacteria or crystals.  That may well answer your "why is he doing this" question.  If it doesn't, you can then look for behavioral reasons.

First, welcome to The Cat Site, though sorry it's under these circumstances.
The fact that he has lost weight and began peeing on everything are, to me, two red flags. You're right to be concerned; you're right in that he is trying to tell you something is wrong. If he were just peeing inappropriately, I'd chalk it up to a behavioral issue (after of course ruling out a UTI). But you said he is noticeably thinner: that could be anything from cancer to kidney problem to diabetes. How old is the cat? Can you tell if he is eating, but still losing weight.
I would take him to the vets asap. I would impress upon the owner of the cat that it is serious; that she is "inattentive' also worries me also.

The cat needs to have his thyroid checked! My cat was losing weight but eating fine. She started pooping outside the box. My husband was hospitalized and the pooping escalated, I couldn't stand it so had her taken to the vet to be boarded so I didn't have to deal with it. The vet examined her and found her thyroid to be very large; her level was at 18 when normal is 4! She was down almost 2 pounds from the prior year (she's always been thin but fluffy-haired so it wasn't too noticeable). We opted for surgery, it wasn't cancerous, just enlarged, and she is fine and no more pooping!!! She has gained 2 pounds in about 3 months. Hope your solution is this easy.

She is very loving to him when she is home and he always has food and treats, but I feel like she has lagged behind on this health issue.   He's had the weight loss issue for a while now.  She had his fecal matter tested, which came back negative for anything, but never did thyroid testing, I think she either didn't want to spend the money or didn't have it.  He's 12 years old and has been eating a lot, but still really thin.  The pooping has been going on for a while but it has suddenly become more aggressive and more often.  The peeing is new.   I have been disappointed with her attitude lately, I had to text her yesterday b/c he had been out of food for two days.  Don't worry though, I gave him some of my cat's dry food and our other roomie gave him some turkey.  He's definitely still getting attention, but I do hope she took him to the vet today.  I'm at work, so I have no idea.  I do think it's possibly his kidneys, thyroid or a UTI. Everyone on here is so great, thanks!

At 12 years of age, he is considered 'senior'--and this is the time some cats develop senior like problems. A simple, releatively inexpensive blood test will tell you a lot. And if thyroid, the treatment is usually a pill a day.
One sometimes key factor to whether it is a UTI is the amount of urine. For example, when Ritz had a UTI, she peed in the litter box but only very small, tiny amounts.
Good luck, and do keep us posted.