Hello! Your senior cat will need to have some dietary changes made and feeding an exclusive all wet diet is the best step.
Before I address the issue of feeding though, you may want to try pureed pumpkin as suggested by the other contributor. Make sure it is can pumpkin or fresh pumpkin, and not those used for pie fillings. Pumpkin will help a great deal when it comes to softening the stool and allow fecal matter to be excreted properly.
A senior cat would need a high protein and nutrient dense food. As for the requirements of more calories or otherwise, various studies conducted failed to come to this conclusion. While middle-aged cats face the risk of obesity, older cats start losing weight naturally. Also, as cats get older the digestion process slows down, and the cats need food that is easier to digest.
Some studies have shown that senior cats do not digest, and thus absorb fat, as well as younger cats. This means that older cats may need to consume fat that is more digestible to get the same amount of energy. The protein needs of cats are higher than the protein needs of many other animals. Inadequate amounts of protein in the diet can impair immune function. Unless the cat has a health condition which would call for protein restriction, an older cat should not be placed on a protein restricted diet.
Also, when you make your decision to purchase cat food, always learn to read the ingredient labels. They tell you a great deal what the food is made of and if the source of meat is optimum. Grocery brands are never a good choice.
You want to feed a food which is high protein - meaning the meat source must come from wholesome muscle meat. Make sure to select only food that list chicken, lamb, rabbit, beef, turkey, venison, salmon, whitefish, herring or a combination or those and not one that says by-products (chicken by product, meat by product, fish by product)
Stay away from too much fillers like corn, corn gluten meal, wheat, wheat gluten, soy, oatbrans, etc. These are all cheap filler which help reduce the cost of manufacturing but brings little to no benefit to your kitten.
You would also want to stay away from preservatives like BHA/BHT and Ethoxyquin which are proven to be harmful to your kitten. Also you do not want other artifical preservatives such as propylene glycol and propolyneglycol .You need natural preservatives such as Vitamin C, Vitamin A or Rosemary.
Your cat need an essential amino acid called Taurine. Cats can only receive this from eating wholesome meat. Choose a food that contains ample amount of Omega 3 sources - Salmon Oil, Flaxseed Oil or Canola Oil.
There are a few high quality cat food that is suitable for your cat and they are formulated for all life stages.
Hope this helps and best wishes to you and your wise cat!