When a cat starts eliminating inappropriately, it's either due to a medical condition like a urinary tract infection, crystals (tiny particles which develop in urine that is not acidic enough), or the cause is behavioral.
First, you need to rule out the medical reason, so your cat should be evaluated by the vet as soon as possible, for a definite diagnosis and treatment, if necessary. Antibiotics will be prescribed for a urinary infection and dietary changes and/or nutritional supplements to acidify the urine, will be suggested if crystals are found. An immediate vet visit is even more important for male cats, as their urethra is so narrow, a urinary blockage can occur very quickly and then this would become a life-threatening emergency. Because your cat's urine is so thick, he might be dehydrated, his urine may contain crystals, or he may be experiencing another medical problem, like kidney failure, which is causing this unusual consistency in his urine.
If a cat experiences pain upon urination or defecation, he will come to associate the PLACE of eliminating--the litterbox itself--with that pain, and then choose to eliminate ANYWHERE else. Cats in this situation usually choose 'soft' materials to go on, thinking this will avoid the pain. Carpet, throw rugs, bathmats, clothing, backpacks, sofas and beds are usual favorite places.
In order to prevent your cat from returning to the places he's already soiled (if he smells it, he'll keep thinking they are appropriate toileting places), you can use an enzymatic cleaner to completely remove all traces of stain and odor. Nature's Miracle, Zero Odor, and the additional products below, are good for this purpose, and can be found in most pet supply stores, and online. For more info and to view all related products, please see:
Cats can also sometimes start eliminating inappropriately if there has been any kind of change in their lives or environment, as this causes stress and anxiety. If the reason for eliminating outside the litterbox is NOT found to be medical, it would have to be treated as behavioral.
Cats are creatures of habit and do not take kindly to change. If you have had any new people added to your household, or if a 'usual' person has left, if you got new furniture, carpet, appliances, another animal, etc., or if your usual daily hours at home and/or away from home have changed or you've gone on vacation recently, any of these situations could cause your kitty stress.
Feliway/Comfort Zone for Cats is a cat appeasing pheromone which can help to relax stressed cats and curb urine marking and/or inappropriate elimination. It comes in both a spray and plug-in diffuser. The diffuser plugs into an electrical outlet like an air freshener, but has no scent to humans.
For more information, please see:
Major pet supply stores carry Feliway, you can also order it online, and some vet offices carry it.
There's another product which I think may help you get your kitty back to using the litterbox--it's a clumping kitty litter called 'Cat Attract'--it contains herbal attractants which encourage use of the litterbox. It too, may be found in major pet supply stores.
You should have more than one litterbox, if you don't, already. Some cats prefer to urinate in one box and defecate in another. The rule of thumb is usually one box per cat, plus one. If you have a multi-story house, a box on each level is also appropriate and usually prevents accidents. Of course, keep all boxes immaculately clean at all times. Scooping/cleaning the boxes once a day, as opposed to once every other day, will be necessary.
Keep in mind that first you should bring your cat to the vet to determine if the cause of eliminating outside the litterbox has a medical basis.
I hope all goes well, and this problem is resolved for you and your cat, very soon. An older cat who is eliminating inappropriately will not be easy to re-home, and I hope it doesn't come to that, but if it should, call some cat rescue organizations in your area and ask if they will take him and/or help you re-home him.
I wish you the best of luck in helping your kitty.