I didn't read through everyone else's posts, so forgive me if I'm repeating what someone else said.
Bloody stool isn't something to worry about if he only has it occasionally, unless it's in great amounts. If it is a constant thing you my want to have your vet check him. Both animals and people too will get bloody stool when they have an irritated stomach. The irritation could be from constipation or diarrhea, something he ate could have caused it, or even nerves.

Probably more information than you need but... stress and nerves cause me to get blood in my stool. When I travel away from home, when one of my pets is sick, etc. My stomach always gets butterflies and then I get bloody stool along with it.

I'd just keep an eye on him. You may want to even mark it on your calendar so if it happens again you can look back and see how long it has been since the time before.