Did your vet do a biopsy of the lumps to confirm his diagnosis?
The symptoms sound similar to those of my piggy, Peppermint. She has lymphoma, cancer of the lymphatic system.
You are right, there is no cure but you can give them an extra couple of months with steroid treatment. We considered chemotherapy but couldn't find enough evidence that it would have any significant impact on her lifespan or quality of life. It hasn't been been tried very often on pigs.
It is interesting to me that your pig, Rum is the 4th pig that I know of on here to be (possibly) diagnosed with lymphoma in the last couple of months. I thought cancer was rare in pigs. Could this be caused by a virus or is it just that we are more vigilant and have better vets or am I just paranoid?
If it is leukemia and not lymphoma, I believe they can still live a good long life but with a compromised immune system. Pebble has a lot of knowledge on this, I'm sure she wouldn't mind if you PM her, she is lovely.
If it is lymphoma, the steroids will improve the quality of life. This has certainly been true in my Peppermint's case. Although I know she is dying, she doesn't and is still a happy little piggy enjoying lots of extra treats. Long may it continue!
I am so sorry for the heartache this brings and wish you all the best for Rum. x