Torn Between Choices Options

[Caspian's Dad]

My best friend Caspian a mix of Chow, Retriever, Border Collie, was just diagnosed with a mass/tumor on his spleen. I am so torn and confused as to what course to take. He just turned 12 in January, and also has osteoarthritis in his back legs/hip that cause him constant discomfort (he taking Tramadol to help with the pain). So my dilemma now is to figure out if surgery to remove a tumor that has a 75% chance of being cancerous and thus giving him a 2 - 6 month life expectancy. If I do not have surgery for him, his prognosis is anywhere from weeks to months.
I am torn whether to have the surgery to remove the tumor, or to let things run their course (based on his age and life expectancy of the mix of breeds). If I do nothing, the guilt of feeling like I didn't try to help him will be overwhelming. The flip side, is to have the surgery, that will make him feel better right now by removing the tumor, but if it is cancerous, and comes back aggressively as most cases like this tend to. Then I will feel like I have put him through the trauma of surgery just to have to endure getting sick all over again.
My heart wants me to do anything and everything I can to keep him here as long as possible, but I do not want to be the cause of more suffering to him. Right now he is in generally good health (tumor aside). Good color, his blood work came back very good, and I have been told he should tolerate surgery and come through it well. I am terrified of what happens if I do not do the surgery just as I am of the tumor being cancerous and coming back very aggressively.
I wish so much I could just ask Caspian what he wants me to do for him, but when I do he just looks at me and wags his tail, and I start to cry all over again.