Friend, when we touch and visualize with the ease the back bone
it is consistent with a dog that is underweight. In this case, if the bones were not visible before as you already know he has been losing weight. Now, the big question is: Is the weight secondary to not eating as much or secondary to an underlying medical problem? You see, the mass that you are seeing in the abdomen could be the spleen. As the dog losses weight he will also loose abdominal fat making the palpation or visualization of abdominal organs easier. In older dogs the spleen also tends to be larger. So, this does not necessarily means it is a tumor or a mass, it could be normal.
The other possibility is that he does have an abdominal tumor either in the spleen, liver, stomach
or kidney. And, the weight loss is part of the paraneoplastic syndrome where patients with tumors
will loose weight.
We also have to consider dental
disease as a possible reason why he is not eating as much and may be uncomfortable (restless) and losing weight. Or, he could have a totally different problem like arthritis
making him uncomfortable (pain), not eating as much and losing weight.
From the above discussion you can see that there are lots of possibility. This could be as a consequence of dental disease or something worst like a splenic tumor. I would recommend you consider having him checked by family veterinarian. He will need to have some testing done like blood work and possible xrays. I think the mass that you are seeing is the tail of the spleen, whether it is normal or secondary to a tumor will need to be confirmed with xrays.
Please, let me know if you have additional questions.