Just make sure he poops before you leave.
I know with Jackson, every morning when we wake up, he goes immediately outside and goes poop and pee and then he can hold it for about 4-5 hours. And he's 14lbs so his bladder is a bit bigger! I can't imagine leaving him for 8 hours every day expecting him to hold it, even though he is 8 months old now.
Like yorkiepuppie said, if he won't go... make sure you feed him and give him water, walk him around a bit, he will have to go! Just don't leave him in his crate until he gets his poopy out