Poor guy! People are often surprised about this condition - often hard for people to wrap their head around the idea that your own body is attacking your own teeth and gums. That is often a painful situation, and it’s super frustrating that removing the teeth didn't solve the issue (but sometimes that’s the only way you know that…remove the teeth and see if they get more comfortable). At least the accumulation of plaque won’t be there to cause further inflammation.
Here are the 3 things you gotta know for what is proposed:
Prednisone (or prednisolone) is not a pain medication. BUT, it can reduce inflammation, especially that caused by the immune system, and therefore indirectly reduce pain. It also might get him to eat more if he is losing weight.
Metacam IS a pain medication, that also reduces inflammation, but doesn't do anything much to change an over activated immune system. I've written about my experiences with Metacam here: http://www.iwillhelpyourcat.com/videos/#/metacam-cats/
Tramadol is a pain medication that changes their perception of pain, but won’t really reduce inflammation at all, or effect what the immune system is up to.
So you have three things at work here. You have the sensation of pain (which happens in the brain), you have inflammation, and you have the immune system. All working together to make this little guy miserable.
So what I would do is ask your cat what his preference is. If I really wanted to see how he felt about it, I would do a separate trial with the Metacam and prednisone (they never can be given together though - only apart separated by a wash out time). By doing this you will determine if blocking inflammation and the sensation of pain is his priority for comfort, or if dropping the immune system response in the very way that prednisone does is what matters most for him.
Once I found what his body preferred, I would put together a strategy to minimize the particular type of side effects that each one could have. So I would say “this is what your body wants, now how do I make the most of the benefits while keeping any side effects far away”. This means I often find other things to give concurrently so I could minimize the doses I was using. Or cycling them on and off the medication. Or giving other medications or treatments that increase the safety factor by reducing side effects.
Because of these techniques, I don’t see a lot of side of effects to either prednisone or Metacam. I know it get's written about on the internet a lot (especially with Metacam), but I find in practice, it's all in how it's implemented. It can happen of course with the best of planning, but it’s not luck that it doesn't happen often. I try as stay away from high risk methods of use: On high dose prednisone (immunosuppressive) twice a day for months on end in a fat cat? Could be asking for trouble. Using Metacam daily without watching their hydration or testing their blood to see if they have stable kidneys? Not a good idea. That sort of thing.