I have seen a snake with a growth just below the spur by the right side of the vent. I know that particular snake has had it most of his life. As far as I know it has been just dismissed as a odd growth and has not been removed or altered (tested) in anyway. It does change size becoming smaller and larger over the years. This snake is 9 or 10 years old and has lived quite 'happily' with this for most of these years. If it is cancer I can't say. I sounds similar I might suggest leave it until it becomes a hinderance or is anything other than closed (no bleeding puss or anything like this.)

Slow to no weight gain it is quite possible that that is related to the growth but perhaps it could be internal parasites too? Have you had a fecal exam done? It might be worth it, they often suggest running one and then a few moths later running a second as evidence does not appear in ever deification.

I hope that your snake does well and I wish you the best of luck.