I would split the dose and give a half dose with breakfast and a half dose with supper. If you could switch the dry food to a grain free one that might also be helpful. You should do it gradually mixing the two foods together over the course of 10-14 days increasing the new food. Older cats might not want to eat something different. PROBIOTICS and OMEGA 3,6,9 can also improve digestion and reduce inflammation. I also suggest a choline B vitamin product such as GeriForm or Cholodin for elderly patients.
Has your veterinarian checked a urine sample for crystals or infection? It would be good to have a veterinary chiropractor check him over. It can help with better movement.
Please let us know how he does.