Oh no. Your poor baby. If you're not going to amputate, no need to do a biopsy. Fungal diseases can be detected via a blood test, titering for cocci if it's possibly Valley Fever. Has he been tested for TBD's? If he's still uncomfortable with just the Metacam, ask your vet about combining an NSAID with tramadol for more effective pain relief. BTW, if you get a prescription for tramadol, it's pretty inexpensive at Costco, Walmart, etc. Sending lots of prayers and light to you and your sweet boy.