Here is a decent photo of betta anatomy http://www.bettaparadise.co.uk/fpdb/...my_Diagram.jpg
now I don't think that would be the swim bladder to me. It sound like some kind of sore or something. Has it grown larger since your post? Here is a picture of a cyst on a goldfish http://www.fishpondinfo.com/photos/f.../jessfish1.jpg
Sometimes this could be an abnormality in your fish's genetics or perhaps there is an underlying bacterial infection. It could also be due to a reaction to a parasite. If his lump appears to fill up with fluid maybe you should salt bath or treat with medication. As for cancer...well fish can get cancer, but I think this is probably not the cause of your lump from your description.
I hope this helps you try to figure out what it could be!