Doing a biopsy is definitely an option. It is also possible to surgically explore the foot although there may end up being more damage done. There is a good blood supply that goes to the foot and often it takes longer for this area to heal due to the pressure that is applied to it on a normal daily basis. The problem with a biopsy is that if whatever is causing the problem is very small, finding the correct area to biopsy would be hard. It may only be a very small place but because he is licking it so much it makes the whole area inflammed which makes it hard to determine where to do the biopsy. The other possibility is that there has been some type of nerve damage higher up in the leg which is causing a sensation in the foot and making him lick and bite at it. You may want to consider having xrays done of his shoulders and spine around his neck/thorax area. Them may show something that is affecting his foot.
I have to call it a night. Consider soaking and other xrays.
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