Thank you for your patience. Buddy's symptoms are extremely serious and given he is a middle aged boy, I would be very concerned he could have a blocked bladder. If he hasn't urinated in the last 24 hours, then this is highly likely. A blocked bladder occurs when crystals in the urine block the urethra (penis) meaning he can't pee. This has an effect on his kidney function pretty quickly which would lead to severe lethargy along with the inability too. A blocked bladder really is a medical emergency, so as this is a real possibility here - you really do need to try and get your boy seen immediately by your local vet or ER vet. Other possible causes for his symptoms include anything from a primary kidney or liver issue, a toxicity or poisoning, or potentially even trauma if he is an outdoor boy.
Given the severity of these symptoms, there really is no time to waste. Don't attempt to treat him from home - he really needs to be seen by your local vet or ER vet straight away. Here they can immediately palpate his bladder and get an understanding as to whether this could be a blocked bladder. If it is - they will need to sedate him and unblock him straight away, place a catheter, run some blood tests to check his internal organ function, give pain relief and start him on an intravenous drip.
Please do what you can to get your boy seen straight away for the above reasons. If you have more questions or if I can help in any other way, please do not hesitate to ask! If you would like to accept my answer, please press RATE OUR CONVERSATION (I am not compensated in any other way). Bonuses are always welcome. Thanks! I hope to work with you again soon!
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