Hello From Canada

Hiiya
I've just joined this forum to get some outside insight on my pooch!
Not sure if I should post it here? but I am.
So my dog is a Maltese Terrier Poodle and he's now 15 years old. So of course for the past while we've been mentioning every now and then that it might be time to take him to the vet and say goodbye. But I feel like it's too soon. He's still very much got his dog personality. But he does spend most of his time sleeping, he only eats one meal a day and right after goes out to poop. His hearing and eyesight don't appear to be as good as they used to be and his can't jump up and down on couches and things anymore. He's gotten quite bossy though and let's us know when he wants something (sometimes we think even he doesn't know what he wants). So how do we tell when it is time to let go? I don't think we could take him to the vet just to see, it's gonna be a one time, one way journey. Any thoughts or help would be really appreciated!