It does look like he'll probably lose it, I'm afraid, but he'll be fine. If the bone is exposed the vet ought to put him on a course of antibiotics, probably 2 weeks.
Is your vet experienced with reptiles? If the appearance of the damage has changed significantly then it's worth going back to the vet, if it needs amputation, the sooner that's done the better, to minimise chance of an infection taking hold.
Here's my tripod with his nipped tail, I adopted him in August from an animal shelter who rescued him from a hoarder.