My son was completing his taxes yesterday and end result was he is owing roughly $500. He went to use his Unused Credit and it "automagically" took his full amount of credit and he is ending up with roughly $3,000 to the good. 

He would like to utilize this feature to bring his net owing to $0 and maintain the remainder of his unused for a later time. 

So first off, can he do this? (I would think so) And 2nd, how does he go about changing the value to the appropriate amount?