I am so sorry to hear about your pup.
There are many reasons not to just tie of the mass, one of which is that it can be severely infected and be a big health challenge to your pup. In an animal this age, something more severe than a skin tag must be considered, especially if this mass is the size of your fist which seems a little large for a traditional skin tag.
I certainly understand money issues being tight. There is the possibility that with an office visit and a simple test called an aspirate and cytology, it may be possible to get a more precise idea of exactly what this mass is. It is important to do this because it will certainly help with giving you a prognosis and will help inform you of what to expect in the future.
With something like this, there are a lot of factors that can help give you an idea of how bad this mass might be. One of the things to consider is how fast is it growing. Are there masses anywhere else? How long has this mass been there? Has it progressed significantly?
BotXXXXX XXXXXne, given his age, and the size of this mass, he should see a veterinarian. With just a little time and not too much money you may be able to get a lot of information that will help you deal with his health as time progresses.
Best of Luck! Please let me know if you have any more questions.