Hello,
Wow that looks like probably hurts him some. It is very possible it is fungal but could he have
burned himself on something?
As suggested, consider the anti-fungal cream, or you could try some raw unpasteurized honey
on it also.
Be sure to keep the area clean daily & if you are using a loose substrate I would recommend
putting some soft fleece towels down for him in case it is painful for his tummy right now.
Tracie