7 Answers

Tips here...http://www.wikihow.com/Cure-a-Cat-of-Constipation

don't just give him dry, hard,cat food all the time. Give him a little dry food off and on thorughout the day, then once, everyday,like in the evening, give him a can of moist, soft, cat food . Keep a water dish for him not only on one floor, but in the basement, too, or upstairs .

 My cat has never eaten anything but dry cat food (and whatever she caught when she was an outside cat) and has never had problems with constipation.  Your kitty may have a serious disorder; I lost a favorite feline to a kidney blockage in a matter of hours.  I'd suggest you make an appointment with your veterinarian, just to be on the safe side.

My sister and her husband give their dogs about 1 tablespoon of canned pumpkin and a dash of olive oil in their ALWAYS dry food.  Not sure this would be advisable for a cat, but it's an inexpensive fix that works for them.  
The problem I see with the pumpkin is all the stuff that's been added to it.  

A teaspoon of veg. or olive oil mixed into the dry food will help & also helps with shedding and dry skin etc...

Get a dog!

yasseen - quite painful for your cat - check his urine - colour if possible and smell important - if its smells at all especially smells of ammonia - also check both urine and faeces for blood which I am sure you have done (ammonia and blood - kidney disease). You will know if he is in pain otherwise, or loosing weight. Also could have a problem with his anal glands - If they are enlarged can be felt with a small gloved finger by vet.  All the other advice given here will help - get the constipation sorted and then see how you go.  

Don`t assume it`s constipation!..Cats with cystitis and urinary problems can give off similar body actions/symptoms to those of constipation...Cystitis and UTI`s IN cats..especially Male cats can be fatal...In any case I`d get the vet to examine the cat and treat ASAP!

Good Luck.