Sorry about this beardie’s predicament. There is no way to tell what the problem might be from one photo.
Please bear in mind that I am not a veterinarian. Let me ask a couple of questions.
Is this the only swollen limb?
Is he eating normally, or is he off his food?
Has he eaten anything unusual while you were gone, such as wild caught food?
Was his enclosure kept clean while you were away?
If this is an injury and is becoming infected, he will definitely need an antibiotic injection. In the meantime, the foot should be kept dry, out of sunlight, and covered with an over the counter triple anti-biotic cream, twice per day. It may be too late at this point for a topical to do any good, but it is something that may do some good and no harm. Be sure that his substrate is freshly cleaned and allow absolutely no feces to be laying around in there for hours at a time. Remove them immediately and be sure to check for others each time you apply the antibiotic.
I hope this was helpful and I will check back for your answers again this morning.