What vessel did they say it was wrapped around? If it isn't ruptured at this point, getting it out would actually be a better plan now before it has a chance to spread or rupture and seed the abdomen with it. We tend to see a lot of these kind of tumors where I work, and honestly his age wouldn't be a stopping gap to us for surgery. We routinely do surgery on older dogs than he is, and generally speaking, the vast majority do pretty well. It depends on the mass in question, the dog's health status at the time, things of that nature.
If it is something that is wrapping around a major vessel, it may be something that you want a board certified surgeon removing, since they have the extra training and generally more specialized tools to deal with that kind of situation. I can't tell you how many times I hear from the surgeons that they are having to be cautious of the aorta or vena cava in the midst of removing a tumor or something else from a dog. It definitely makes us all be on high alert and ready for something to happen, but I think it is rare that they actually have torn one of those vessels on accident.
Since the mass is in his abdomen he would need to be asleep if they were going to try to get a biopsy of it. Given where it is located, if you are able to afford and want to go through surgery, having a CT with contrast would likely give them an idea of how easy or hard it would be to remove, given the proximity to a large vessel. Just to give you a little more food for thought.
Good luck to you and your boy! My parents lost our golden last March to hemangio. His had ruptured, spread to liver and lungs, and while he recovered from surgery, a few days later he passed away.
Meanwhile others who had them rupture recovered well, and are still alive. It really all is a game of chance. :/