Went to a pet store which sells another species of Asian toad (Bufo gargarizans) and had a chat with the shopkeeper there yesterday. Showed him the above pictures and he said the darken region on his belly wasn't a big concern. He guessed that he stopped eating because of the lowered temperature initially, and after we turned on the heat lamp, it was probably too hot so that he continued to refuse food and soaked in water all day long. We then turned off the light and he seems to be more active now. We then tried to offer him a cricket and he finally took it!


Anyway we will still keep an eye on his weight to see if he's really fine. I hope it turns out that it's just me worrying too much!