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Since Jimi is straining to defecate and has vomited potentially causing the loss of fluids and dehydrating him further, the best recommendation I can make for him is to take him in to be seen tonight. The vomiting is probably from his straining and continuing to strain is likely to cause more vomiting. However if that is not possible he should be seen first thing in the morning. Constipation secondary to dehydration is not uncommon among cats with kidney failure and if he has a history of this, then it may indicate a condition called megacolon that results from poor colonic motility and causes chronic constipation. The pumpkin is good as are the fluids you gave him but he may need to be manually evacuated or given an enema to relieve the constipation he is experiencing tonight.
Unfortunately, the over the counter enemas that you can find in the drug store for people are not safe for cats and can cause life threatening electrolyte disturbances so I cannot recommend them for home use. Your vet can give you the supplies to make an enema at home (and teach you how to do this safely) or give you a specific recommendation for an over the counter product that is safe if Jimi needs to have this done regularly.
In the longer term, there is a medication called lactulose (prescription only) that Jimi can be given to help loosen up his stool and make it easier to pass as well as continuing to add the pumpkin if he will eat it or trying 1/4 tsp metamucil with his food.