I wouldn't be to quick to put it down. Dragons do just fine with no toes, feet, and even whole legs. Most manage fine, and can be perfect pets. If you go the adoption route you'll find lots of people who look to adopt special needs pets, and most of these people accept, and love their pets even more for their slight disabilities.
If the infection does not heal and spreads there can be attempts to remove dead tissue and try again at the healing process. Did the vet prescribe any oral antibiotics? What exactly is his treatment? I would suggest keeping him on paper towels if not already. Maybe separate from siblings so you can keep his foot even cleaner.
I can tell you care about your little guys, and I'm sure you'll know if treatment is going successfully or not, and at what time a decision has to be made for him. Just wanted to give you some hope, and some ideas. It's a shame when it happens. you can feed 3 times a day plus greens and still get a nip. sometimes it happens when feeding, and one mistakes, misses it's mark. I even had one this year right before my eyes. One was eating a cricket and it's sibling went over his back to get another cricket, and what do you know sticks his toes in his clutch mates already chewing mouth!! That fast and couldn't do a damn thing about it. Just up the feeding, separate to give more room as needed, and the chances for accidental nips like this decreases.
Good luck with the little one.
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