... not completely sure I understand... If the other person is on the deed then they already own that house
That is correct we are both on the deed but the mortgage is in my name. He is refinancing the house to buy the house from me for the remaining balance of the mortgage.
OK, then no, you are simply gifting him your ownership in the home
Do I need to have a document that he signs that states that for tax purposes?
You'll have to retitle, convey the property somehow, such as through a quit-claim deed ... that will be documentaion enough