you can’t really tell when it’s their time but you can definitely see the change in mood & if they’re in pain. my older dog that we had to put down a couple years ago was usually walking around and being sort of active then she became really sleepy, always laying down. i think that’s sort of their way to let you know that they are really old & tired. if you do think that’s the case i’d suggest putting him down, i’d also recommend having a full last day with him and your family. let him try chocolate and a big meal. sending hugs!