Hi
sparrow - hows he doing?
Minor weight fluctuations aren't normally anything to worry about - especially in old age and on medication - and like you said - he is still active and eating. I recently had two very old gerbils who naturally declined in weight due to their metabolism and lack of activity - as although they were still active when they were active - they were active much less often.
Also, I have had gerbils with SGT who - when initially put on pain killers made a huge improvement. The painkillers at first stopped them noticing their tumours and so they stopped chewing them - but eventually they got either immune to its effects or the pain increased for some reason.
Additionally - the abcess could have affected his fluid circulation and so the weight change could just be in imbalance of 'water' in his system after the amount of fluid in the abcess itself was removed?
Anyway, we will never really know what they are feeling, so we can only really gauge what WE think they feel having known them for some long. You have known them the longest - so I suppose only you can decide when he is still ok and when he isn't himself anymore.
I haven't had any experience with an abscess or tumour in that area myself - so can't help advise on cleaning or healing that, but I have had plenty of older gerbils and plenty of gerbils with SGT or ovarian cysts - so I know about working with my vet and making decisions along the way. Sometimes for one more thing, one more treatment - sometimes to stop treatment - and sometimes when palliative care isn't working anymore.
Cancer in of itself isn't something regularly talked about on the forum (SGT and cholesteotoma are though) so I am not sure if that would be more likely than and abcess - perhaps you could do a search on this forum catergory for both abcess and tumour and see what people have been saying over the years? Maybe that could help.