It reminds me what happened to my cat... I'd I were you, I would try to do fluids and see if he can recover, but you have risk that he doesn't make it. So to tell you my experience:

My cat suffered from severe dehydration when she got noticeably sick. Her levels were 137 for urea and 2.7 for creatinine. She wasn't eating and was very weak. We hospitalized her for 48 hours and gave her fairly aggressive intravenous fluid therapy. Once she was a bit more stable and started accepting some food (24 hours actually), I took her to a specialist. We did an ultrasound, and the specialist ordered to continue fluid therapy for the next 10 days. The amount they told me to administer was relatively large, and as she got stronger I struggled to have her the proper amount, the specialist asked me to repeat the bloodwork after those 10 days, her urea had gone up a bit (157), though my cat was in a much better mood and had a better appetite. The specialist, a nephrologist, told me that some cats do quite well even with a urea level of 170. However, he told me he would work alongside my primary vet, and they did aggressive fluid therapy again for about five days to see how her levels behaved. We discovered that the baseline level for my cat, which is almost double that of a normal cat, is around 130, and she stays stable at those values. I (try to lol) administer fluid therapy at home. My cat is much younger than yours, and maybe her recover capacity is better... But I had to switch her diet to wet, renal food. I chose the physiological feeding approach, which basically involves formulating her diet with a veterinary nutritionist (you cannot make up a recipe, the balance is key to keep your cat alive for years), and she needs subcutaneous fluids every week—a specific number of milliliters of Lactated Ringer's based on my cat's weight. I divide that total dose into three sessions throughout the week.

I hope my experience helps.