If it had been there for years it is probably not an STD. However, I wouldn't take this guys word for it. Go to your family doctor and tell him/her what happened and what, if any, risks you may be at from kissing someone with an unidentified lump on your tongue. They will be able to give you an examination. You may not have needed to go out and get and HIV test, since I doubt you would be at risk for HIV from a kiss.
That being said, it sound's like you may also have OCD. It sounds like you are worrying excessively and irrationally. Read about OCD.
I always ask anyone I get intimate with before hand, if they have any STDs. If they are nasty about answering you, then they are not right for you. Most people will be open and up front about it. That is the kind of person you want to date.
Good luck.
Zan
A single lump on the tongue doesn't sound like any STD I've ever heard of. It does seem strange that he doesn't appear to know how he got it. I have a lump on my tongue from falling down stairs and biting it when I was small. It sounds to me that you're a pretty anxious guy but there's really nothing for it but to wait for the test results. Sorry I couldn't relieve your mind more. Let us know if it turns out it is something so we can all educate ourselves.