Forget the word God, call it the "Universal mastermind", his argument is that for something to be objectively moral, meaning that something is always considered right or wrong, there need to be someone to measure against, one that have decided which it is. He is a Christian, so he uses God, but in theory you can use anything, which is why you shouldn't be caught up in him using God.
If there is nothing to measure what is morally right or wrong against, then what decide what is objectively right or wrong?