I'm honestly not sure, but given how difficult most infections seem to be for rabbits to shake, I wouldn't get discouraged about a lack of results after only three days.
Cursory research suggests that the majority of cranial abscesses in rabbits are caused by bacteria that are sensitive to penicillin (specifically, Pen G (aka Bicillin))... though most sources seem to point to this sort of abscess being difficult to shake without surgery being part of the treatment... what I'm gathering is that your rabbit's current line treatment is kind of a coin flip - it might work, it might not (but it IS a viable thing to try and if successful, it might allow you to avoid surgery).
I suppose the pressing questions at this point are:
~ What type - specifically - of injectable penicillin is he getting?
~ How big/severe is the abscess?
~ Has a culture been done? (and if not, does your vet intend to do one if you don't get results from the penicillin?)
If you do end up opting for surgery, I would make sure they do a full blood work-up first - while it can't provide any guarantees, that can give a good indication of whether or not there are any factors beyond age that would make surgery riskier than usual for him and help you make as informed a decision as possible.
Best of luck in treating him - I hope the penicillin helps!
ray:
Here's a link to an article on abscess treatment:
http://www.bio.miami.edu/hare/jawabscess.html
Also worth noting - the author of that article (who has a stellar reputation in the world of rabbit medicine and very impressive credentials) answers rabbit questions for free through
this website. She's a devoted bunny-slave with a PhD in Biology and a *really* nice person (I've chatted with her briefly via email, though not about a health issue) - she devoted herself to being an advocate for rabbits after losing a pet bunn due to an un-rabbit-savvy vet's misdiagnosis.
