Yes, Advantage was just Imidacloprid which was safe to use. Advantage II has an added medication that Im not sure is okay. The box also indicates not to use on other animals (i couldn’t say whether original formula also had this on box or not but it was used on rabbits). I think Advocate was also just Imidacloprid but Im unsure if that is even available anymore.
Here is what wabbitwiki site says about Advantage v’s Advantage II
Advantage was reformulated and is now labeled as Advantage II. This new formulation has treatment for other parasites in addition to fleas. Although still generally safe for rabbits, some rabbits have shown more sensitivity to this new formula. Revolution, available only by prescription, treats fleas and mites safely.
The original Advantage formula only contained imidacloprid. Imidacloprid kills adult fleas and has some effect on flea larvae, but it did not kill flea eggs. Advantage II is infused with the insect growth regulating chemical called pyriproxyfen. Pyriproxyfen artificially maintains elevated levels of the juvenile growth hormone in flea eggs and this prevents the eggs from hatching.
Advantage Multi is another option, it contains added ingredient of moxidectin which treats some forms of mites. It may be prescription only though, depending on where you live.
Have you used a flea comb on him at all (putting any fleas you find into soapy water) ? That will help until you are able to find suitable topical med. You could also try using food-grade diatomaceous earth powder, taking care not to get it near the eyes. It won’t act as a preventative but may deal with existing adult fleas.