Cat Is Losing Weight, Fur Etc. What Could It Be?

Your cat is trying to tell you something by going to the bathroom outside the litter box.  Have your cat checked for hyperthyroid.  If he is eating well and has runny stool, this could be a sign.  Also, hyperthyroidism is very common as cats age and can be controlled by meds.  This disease can very serious (long term) as it effects the heart.
Good luck and hope you find an answer.

There can be many reasons behind this problem. Is your cat chewing its hair or hair is lost because of too much grooming? Lose of hair in clumps is an indication of Alopecia. It's important to find out the actual cause behind this problem. In most of the cases it is because of flea bites. A flea bite sometimes leaves such reaction bad reaction that steroids or antihistamines have to be gives. So should first of all make it sure that your cat is completely out danger of fleas. Mites can be one of the reasons that create mange and loss of fur takes place. Ringworms also create this problem of hair loss along with dry flaky skin. Food allergy and inhalant allergy (due to mites or pollen) also cause this problem. So change your pet's food and look for the results. Keep its litter box clean and make it more attractive for it. Watch carefully if it is licking its genitalia frequently.