Wel this is just my pinion, and others may not agree, but I'd say as long as he doesn't appear to be feeling any pain, and is still getting some happiness and quality of life, hang in there with him.
Gradually declining desire for food is typical of the "hospice" -or end of life process -situation in most animals. They slowly give up the things of life which were so needed when they were fit.
That is a sad process to observe, but a natural part of the "letting go of life". I would to some extent let him call the shots. I would be careful about force-feeding, as it would upset him I'm sure. If he will tolerate it without any upset, then a kind of "soup" made with something like babyfood (containing NO onion!) with a little water, or cottage cheese whipped up with some milk or water in a blender, will make a consistency suitable for syringe-feeding. For syringe feeding, you'll need a needle-less syringe (probably available at a pharmacy or at your vet's) and just put it in at the corner of his mouth at the back. Don't squeeze out the contents too fast, as he won't be used to this form of eating, and too much at once could go down the wrong way.
Just make sure whatever he DOES manage to eat is highly nutritious, easy to digest, and fairly small amounts at once.
Fluids however are important. Without them he will go into kidney failure. So the best way to encourage him to drink is make thin stocks which chicken or fish, and maybe some vegetables have simmered in. Also water in which honey or maple syrup has dissolved are tasty too.
Yes I'd say do try and get something down him, but like I said, don't push too hard. He will most likely show you if or when all this really is too much for him, and he is ready to let go. While he's still "himself" in many ways, still having some -even a little -enjoyment out of life, he's probably not ready to give up.
Sorry to say this, but you need to put your dog down (out of his misery). He's quite obviously dying right in front of your eyes, so I'm not certain what you're waiting for.
Please don't take me saying that as being insensitive, but you're asking us if it's more HUMANE to push food down his throat and force-feed him, and infind that a little ironic (your use of the word "humane").
Cancer of the spleen is incurable, and extremely fast acting.
He's not eating.
He's notmdrinking.
He doesn't want to go for walks.
Am I missing something?
Please, do the right thing. Do what HE'D do for YOU if the situation was reversed.
He's had a good life and you've given him a good family... Let him go.
I'm truly sorry that you're going through this.
LMNO