I'm glad there seems to be an improvement with his stools. I would call the vets tomorrow if it continues to be abnormal. I'm not sure all what could have caused the abnormal stool, but if he continues to improve and overall appears healthy by tomorrow, I wouldn't be so concerned unless it happens again.
It wouldn't hurt either way to jot down the date, description and anything he was offered to eat in the last two days (yesterday-today). Keep the note to see if you can find anything of a pattern if it happens again.
Please keep us posted on how he's doing.