Experienced rabbit owners?

You may not like to hear it but it was negligence. You had 2 months to take him to a vet and you didn't. 

I have had multiple rabbits and when anything is off I took them to a vet even if their energy levels were normal and they were eating. Rabbits don't just start having smaller poops and diarrhea due to old age but rabbits are prone to digestive issues that can turn deadly. Small poop and diarrhea is a sign of that. 

The common reaction is to get defensive but the right thing to do here is to acknowledge that you messed up and take this as a learning experience and make sure it doesn't happen again. I am not trying to say you did this on purpose. Negligence can be accidental. If you are ever worried its not necessary to see the vet you can always call them first and ask. 

If you refuse to acknowledge that you did anything wrong and will repeat this then never get any more pets. 

Losing weight alone is a really good sign he needed to see a vet whether he was eating or not. I know tone can be hard to tell over text but nobody here is trying to attack you. We are just saying it like it is. You wanted to know if it was old age and were looking for other opinions and that's exactly what you got. Rabbits do not die of old age at 6 years. They live 8+ years. You should have brought him to a vet when you first noticed something was wrong. Many people have made this mistake before. Like I said, learn from it instead of being defensive and pretending you did nothing wrong or do not get any more pets. 

It was not old age. A rabbit which is well cared for has an expected lifespan of 8-12 years (and sometimes even a bit more, I've had rabbits live as long as 15-16 years old). Digestive symptoms in rabbits, such as those your rabbit was exhibiting, are considered very serious, and warrant immediate veterinary care, even if the rabbit is otherwise behaving normally. The specific underlying condition, and whether a vet could have prevented the rabbit's deterioration and death will now never be known for certain. The immediate cause of your rabbit's death was your negligence in failing to seek appropriate veterinary care for a sick pet. Please don't own pets in future if you are unwilling to seek veterinary care for a sick animal.

Add: Yes, rabbits are certainly very different than cats or dogs. I would know, I've owned plenty and I have fostered probably over a hundred of them for the House Rabbit Society over the last couple of decades. They are more medically delicate, and, as prey animals, they will do their best to hide symptoms of illness until they are physically incapable of doing so. This means they will very often behave relatively normally until nearly dead, and that symptoms like diarrhea, smaller feces, increased gastric noises, and muscle wasting are all extremely worrying and warrant immediate veterinary care, even if they are otherwise acting normally.