awww what a sad sad story! I tend to agree with Foschi....I think it was a combo burn/rot and it got infected. How terrible for your son but what an amazing lesson in responsible pet ownership. Do whatever you have to ....to try to keep the beardie. Beardies take a lot more to take care of. If he wasn't taking care of the snake I can only imagine the beardie.
One thing I did see while I was reading through this thread !!!!
DO NOT put a snake with any type of these issues on reptile carpet!!!!!!!! Use newspapers or paper towels only. Personally don't use carpet ever...but especially when these issues are present. Carpet harbors bacteria and when you have an open wound such as this...bacteria can and will lead to infections.