Get an abdominal ultrasound asap. Go in as an emergency if you need to get in front of the line. Will cost you more, but you get in sooner. Do not wait for referral.
If there is a mass, then need to search the rest of the abdomen and especially the thorax to see if spread.
The US doc can and usually will offer a needle biopsy of the mass US guided. This may shed light on type of mass.
They will need his previous blood panel for the info, and likely will run a coagulation panel first.
If the mass has escaped to other areas, prognosis is bad.
If contained within the spleen, better odds to get it all.
His pot belly and difficulty in respiration sounds like it may have already spread.
Good luck.
You have my sympathies.
I pray for better, but we are at a stage where he may head downhill. If it is in the spleen, as I suspect, I will have the spleen removed. It could buy him some time. If it is as the vet suspects a tumor between the spleen and pancreas, we will go from there. I can help slow the progression with his diet, exercise, and medication, but we are just buying time. I will have the spleen removed if there are no signs of any other large tumors. It will buy him a month, maybe two. At his age, in dog years, that is a lot more time than it is to us.