Wow, for some reason I was thinking you were talking about a cut going longways down his belly!

If he's looking at you or following you with his head or tongue that's a sign he's probably alert... It's not 100% but they tend to get despondent and not very alert if there's an infection spreading through their body.

Have you noticed urates (white stuff)? If he's been drinking and passing urates, that's a good sign since that injury would make you wonder about injuries to the intestines.