Hard to say exactly what caused it. I would go through your cage and look for sharp edges. I doubt that it is a burn but I guess it could be if you don't find another potential cause.

I wouldn't be too worried. Just keep a close eye on him over the next few weeks and make sure that you keep his cage as clean/sterile as possible.

I would have some Betadine (available at most pharmacies without a prescription) incase you suspect infection (which I doubt will be an issue). Don't apply neosporin or an other oil based cream as they do more harm than good.