So it is actually in his stomach? Not external.
No I wouldn't operate on that as the risk of infection spreading must be huge
What makes you think it is an abscess? Probably not what you want to here but we have lost several young bunnies to cancer in the stomach. I have forgotten the actual name of the cancer but it is very common in rabbits aged around 18 months
There is a bit about it in FHB book
Check
Holiday Gift Guides and
Michaels Ad.