The cat needs to go to the vet for a check up ASAP. Urinating on soft things often means there is a urinary infection present. The bathtub surface might be attracting your cat who associates the texture of the litter with pain on urinating or defecating.

If all is clear on the health front, then the problem is behavioral. You will need to take steps to retrain the cat to the box. Basically you need to confine the cat to a room like a bathroom, or a large cage with only food, water, bedding and the litterbox. As he begins to use the litterbox consistantly you can expand his space. There is more information at this link:
Litterbox Problems

Cats need easy access to boxes and you may need to put several boxes around the house.Be sure to keep all boxes very, very clean. Scoop at least once a day!

You need to thoroughly deodorize wherever the cat has been peeing. Petastic is highly recommended as is Heaven's Own. It is the smell of the "accidents" that causes a cat to reuse the place.

Check into using Feliway, a spray that mimics the happy scent cats leave when they rub their check against something. You can get it from your vet or from Cat Faeries at http://www.catfaeries.com
If you visit the website, you will see a phone number that you can call and for a fee discuss your cat's problems with someone. The person who does this is very knowlegeable and it may be worth the $$ for you. Feliway is also available in most large pet supply stores. There is also a plug-in with Feliway that can help.

Finally, if all else fails, ask you vet about using an antianxiety medication such as Buspar or Prozac. These very often work, however the decision to prescribe them will rest solely with your veterinarian who will be able to monitor the dosage and side effects.
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Smile, it will make them wonder what you are up to!

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