Limping on back leg, xray showed a lump but vets not sure what it is... assuming a tumour

My 9 year old cat has recently developed a limp on his back right leg. We took him to a private vets and they have done an X-ray on him. They found that he had arthritis in his hip but they also found that he has a lump in muscle under his spine near his pelvis. They admitted themselves that they didn't know whether or not it is a tumour but they seemed to just assume it is. They have just kept giving us metacam for pain relief and joint care tablets (both of which he has been on for about a month now but his limp hasn't improved). I just wanted to point out as well that he has been limping for around 2 months. When we initially took my cat for an examination she couldn't feel the lump. After the Xray she said that if it is a tumour it is an aggressive one to have grown within 3 weeks but it is confusing as the limp has been going on long before this lump has been spotted. He is showing no signs of cancer - e.g. when we took him for the first consultation 3 weeks ago he weighed 2.25kg but after the Xray they said he weighs 2.4kg... for an aggressive tumour as well this seems strange as usually they would lose weight, not gain it. He also hasn't lost his appetite and is eating fine. There are no other signs of cancer either such as vomitting, diorrhea, blood in his stools etc. The vet we saw mentioned that because of where this lump is she said she wouldn't be able to remove it but she did say she might be able to remove some of it but would need to do a biopsy to find out what it is and if it is a tumour, what type. I just want another opinion on this matter as everything is very confusing and as my vet said, there is no right answer here. I know that it is advised that metacam is not used long term either as it can damage liver and/or kidneys - which she said were very healthy. I don't want to keep giving him something that could cause more problems, especially when these organs are very healthy. I just want to know what the next step could be as it's not easy to just go on pure assumptions that it could be a tumour. I don't want to give into wishful thinking with this which is why I've mentioned that he's not showing signs of cancer apart from the lump itself. It's just strange as she didn't feel the lump 3 weeks ago but he was limping more than 3 weeks ago. She was just concerned that the biopsy was an invasive surgery which would tell us what the lump is but there's no guarantee that it would help.

You mentioned the current vet did not feel the lump 3 weeks ago, is it possible the lump was missed somehow?

I would get a second opinion before performing any surgery......especially if the current meds he is taking now have the possibility of causing damage when taken over time, that's what I would do.

Are you based in Europe?  Metacam is more often used in Europe than the U.S. for pain relief, that's why I ask.  There are definitely other types of pain relief out there, so you could ask about them.  there is also laser therapy for the arthritis, which is an newer treatment.   As you pointed out, perhaps the lump is not at all the cause of his limping and pain. 

There is no way to tell what the lump is without further checking.  Perhaps an MRI might show something? 

I would definitely get a second opinion before having him cut into, unless you're talking about a needle biopsy, which I don't think you are.  Did your Vet say what kind of biopsy?  Did they run a blood test to check his WBC.  with cancer, I think the WBC  is normally out of whack, but don't quote me on that.   Sometimes, with humans anyway, that's the first clue there is something wrong.