My son did not pay for any expenses during this time or accept any of the rental income
Then who did pay the expenses and receive the income? You? If so, then "you" have to report all the rental income/expenses on your tax return. If it's been for the last 18 months, that means "you" need to amend your 2020 tax return to show the 5-6 months of income received in 2020, plus depreciation and other expenses. You need to do this before you can even start your 2021 tax return so that your starting figures imported from the amended 2020 return will be correct.
