The reason the source of the income is important is that if the son is claiming payment of rent, the parents should be reporting its receipt.  If the son is receiving SSI and he's receiving in-kind income, in the form of free room and board, he may be disqualified for those and other benefits.  On the other hand, if it's Social Security, the parents may be "representative payees" and required to account to Social Security for where the money has gone.  If they tell Social Security it went for rent, they may be asked to prove it from their own financial records.