I would suggest to get a blood test which can help determine whether the mass is cancerous or not. I can't remember the name of it, but I believe it's some sort of generic tumor marker. If it comes back really high, the chances of it being cancer are high as well. If that is the case, you may want to reconsider putting a dog of that age through an invasive surgery from which he is not likely to recover. Hemangiosarcoma (which typically originates in the spleen) is a very aggressive tumor and treatment options in most cases are very limited (as in.. really not much you can do). Also, as someone else has stated, often when you remove the primary tumor, the spread to the rest of the body is expedited.
From what I understand, hemangiosarcoma cannot be diagnosed through biopsy.
In my 12+ yo dog it appeared as a limp, a tumor in his tib/fib (which is very rare). They said there is a small mass in the spleen as well, but recommended to go ahead with the amputation if only for pain relief, since bone cancer is very painful. We went through amputation thinking it's bone cancer (which meant he would have up to 2 years). Then we were told it's hemangiosarcoma.. He adjusted relatively well to being three-legged, but his spleen kept growing and a month after surgery his spleen balooned and he was near death.. I made a not easy decision to attempt to remove his spleen, but asked the vet to check his lungs prior to surgery. His lungs were full of cancer (were completely clear a month prior), so we did not go through surgery and had to say goodbye.
One thing I started noticing weeks before anything appeared really wrong is that his toungue and gums appeared very slightly bluish (slightly enough that the vet didn't think much of it) and he also seemed to be getting more easily tired.
I don't mean to scare you and it's entirely possible whatever your dog has is benign, but I would suggest doing everything that is non-invasive in terms of tests before going for surgery. You may need to go to a oncologist to get the blood test I am talking about, which is what we did.