DDOG1 is 14. He's very slow and old, loves the idea of walks but can't walk for more than 10 minutes before his back legs collapse.
He's got dementia and arthritis. He's almost totally deaf.
He often goes off his food, sometimes for more than a day. He hasn't done this for a few weeks but a while ago he didn't eat for over two days.
He sleeps in a crate and in the mornings when I let them out he often takes ages to just get out of bed and go into the garden, plus he never strays far from the back door.
He walks with a funny gait as he tore a cruciate ligament a few years ago, had it pinned and never really recovered despite therapy.
He has fatty tumours all over his tummy and one on his front leg. None of them seem to bother him, and when I take him for check ups (actually the last time they saw him was Feb) they just say keep an eye on him.
But he doesn't seem in pain, seems quite comfortable with a little slow life and I've just been playing with him and he was almost his old self. Well we wasn't moving around but he was pretending to mouth me and wagging a lot. 
I'm wondering how long I might have left, he's such an adorable dog. I don't want him to be uncomfortable.
How do I know when he's not happy anymore?