I have been studying these "lumps" in the bettas for over 10 yrs now, and I still have found nothing that can be done for it. They are not tumors... I have done a number of autopsies and disections, and it is for sure not a tumor. I can't find any glandular issues in these fish, either, but I have noticed that many of them have issues with their immune systems and tend to get sick much quicker and easier than the others. Whatever it is that causes this, it's commonly found, and nothing treats it. If you notice your fish beginning to lose color, lose a lot of weight and sort of "waste away" I would suggest humanely euthenising it. Once they begin to waste away, it can take months, but they steadily go downhill and you can see that they are suffering. When selecting a new betta, this is something to look for when choosing a healthy fish. It is possible that this is caused by something genetic...
Sorry I don't have better news to post for you. Good Luck to you and your fish!
Dawn