Hi there. So sorry that Indie is having this issue.
There are new treatments for giardia that may be helpful. Your vet may not be aware of these so have them research the following drugs:
Furazolidone
Nitazoxanide
Tinidazole
They will likely have to be compounded by a pharmacy so your vet will need to check into that.
In the meantime, I would practice serious hygiene in the environment like cleaning the litter box DAILY with 1:10 dilution of bleach. Keep the floors where the litter box is very clean with the same dilution of bleach solution. Wherever you use the bleach solution let it sit for 10 minutes before wiping it down.
I would also recommend you increase fiber in Indie's diet. The best way to do that is with a prescription canned food you can get from your vet called Hill's w/d. The extra fiber will help remove any giardia trophozoites from the intestinal lining.
Adding a probiotic like Fortiflora to his diet would be beneficial especially if he has to have a long course of antibiotics. Fortiflora is the best probiotic and you don't need a prescription to buy it:
http://www.vetrxdirect.com/product/view/fortiflora-for-cats-otc/variation-2771Finally, if Indie will let you I would recommend you bathe him 2-3 times a week with a medicated shampoo containing Chlorhexidine. This can eliminate any giardia cysts that may be on his coat - while grooming he may be ingesting these causing re-infection. Lather, let it sit on his coat for 5-10 minutes and then rinse throughly. A shampoo like this one is best:
http://www.vetrxdirect.com/product/view/chlorhexidine-4-medicated-shampoo-for-dogs-cats-otc/variation-10419Giardia can be frustrating and difficult to get rid of :(
I hope this helps. Best wishes!