Fractured foot is now cancer, osteoporosis or myelitis?!

My dog fractured his back left foot on 7/12. He had a check up at 2 weeks and everything seemed fine according to the vet. On 8/21 we went back for the month check up and the vet said the bone integrity didn't look great, and we were told it was likely a bone infection (as well as a foot skin infection that I couldn't even really see) and maybe even osteosarcoma.

Here are his X-rays. https://imgur.com/a/ewhRLSK

He was put on clindamycin and metronidazole, and we were instructed to wash his foot twice a day and given both spray and cream antibiotic. This treatment is supposed to be for 30 days with a 15 day check up which was yesterday. The Xrays showed that whatever he has has spread and now the vet has even thrown in osteoporosis. I don't know what's going on but I think it's clear that the antibiotics aren't working. The vet wants to wait 15 MORE DAYS to do any sort of testing/biopsy. We kept the antibiotic dosage the same and the vet said we can stop with washing the foot. I'm not too convinced about this and even said my worry was that in 15 more days that it would only get worse. I'm going for a second opinion soon but would like to know how this has only gotten worse and wasn't caught in the initial X-rays?!