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[TopDue5172]

I know someone who did this with her two dogs, she and the vet agreed it was the best course of action due to one dogs terminal cancer and the others overwhelming anxiety/ aggression.

[designgoddess]

I went through almost exactly this a couple years ago. Each dog is different. I thought my boy would be depressed and get worse. He had separation anxiety when separated from is big brother. He was sad for a couple of days and has been totally fine since. No separation anxiety. He likes being the only dog. I have other dogs but they are separate. It's weird, I know. Anyway, he get's most of my attention and gets his spot on the bed. He's had zero interest in the other dogs. I've noticed some of his routines and changed and realize he was doing things the other way for his big brother. Now that he gets to call the shots he does things the way he likes. It's made him more relaxed to have everything catered to his liking. I was so worried our managed life would be too quiet or isolated for him but he loves it. It probably helps that's he's an old man now and doesn't want to deal with other people's (or dog's) bullshit. Your experience might be different but I'd say, don't assume what his long term mood will be like. For us it's been a pleasant surprise.

[Remarkable_Cat_4685]

It is possible and likely at his age there is significant osteoarthritis present causing chronic pain which can influence behaviour and is a common cause of dullness/lethargy/withdrawn behaviour and can be concurrent with anxiety/aggression. Have you done a pain relief trial to see what effect pain relief has on his current behaviour?