I absolutely understand not wanting to put him through that, that is why an FNA is so great, you get a lot of information without being invasive. It could definitely be related to the tooth. Oftentimes with a tooth root abscess, the tooth will have no outward signs of a problem. At this point it could also be scar tissue left behind from an insult, that would be why it isn't inflammed.
Dogs also can produce what are called granulomas which are basically an accumulation of WBCs in the tissue, not quite like an abscess. What happens with granulomas is the body over reacts to an insult and pours all of these inflammatory cells into one spot. The cells then become kind of trapped there with nothing really to work on. Granulomas are not a problem unless the dog starts to bother with them, excessively licking or scratching them.
You are absolutely right, if it were infectious, he would be showing other signs such as inappetance, lethargy, or possibly a fever.
The fact that is has shrunk a little is a good sign too. Active tumors don't shrink. Based on what you have told me, you should feel comfortable watching this lump. If it grows, changes shape or color, or he becomes painful or otherwise ill with it, then get it rechecked.
I hope this helps. Please contact me with any further questions you may have. Thank you and good luck!