Blood in the stool can be very alarming but there are lot of innocuous causes for it. Did you happen to see any soft stools? I am guessing not since you didn't mention it but blood can be present when the intestines are inflamed from diarrhea and it goes away once things settle down.
Was the blood bright red or darker? That will give you a good idea where it's coming from - if it's bright he could have something around his anus bothering him but if it's darker, it could be from the intestines.
Anyway, since you didn't find anything obvious in the yard, I wouldn't panic but I would keep a sharp eye on things from this point forward to see if you can pinpoint a cause.