The stimulus payment was based on the 2007 tax return figures but it was actually an advance from the actual rebate which is computed on the 2008 return.
Since he had no tax in 2007 but did have at least $3000 in qualifying income, he received $300.
His actual situation in 2008 is he has tax of more than $600 and so he would be entitled to $600 but since he already received an advance of $300 he would now get the difference (600 -300=300).
If it had been the other way around and he received more than he is entitled to, he gets a break and does not have to pay it back.
taxbilly