Two seperate transactions. 1. The sale of the personal portion the son will not getting any 121 exclusion, since he did not live in the home all and will be responsible for his portion of the gain, if any on the personal portion.

2. The sale of the rental portion; Although the son never benefited from the profit or loss from the operation of the rental property, from the info provided is appears he was a part owner and should report the sale (4797) and any resulting gain or loss shared. Not really fair, but how could he not know of his ownership? he or a reprsentative must have attended the closing. IMO