The 1098-Mortgage Interest Statement will have the name of the first person listed on the loan paperwork as the primary borrower. That does *NOT* prevent your son from claiming that mortgage interest on his own return, provided two conditions are met and he can prove those conditions are met if ever audited on it.
1) He is legally obligated to pay it (and he is if he signed the mortgage paperwork)
2) He actually paid it (and can prove it if audited on it.)
