The scenario you shared has limited details so may not be completely accurate. For example, you don't need to report the gift of a rental property on your income tax return because there wasn't a taxable event (i.e. gain or loss on sale or disposition). You just need to report it on Form 709 United States Gift (and Generation-Skipping Transfer) Tax Return).
See this IRS article for more information about renting to relatives.
There may be information missing about the basis of the property, if you have been renting the property at fair rental value since 2009, then you should have been reporting the rental activity on your tax return and taking depreciation. Depreciation, allowed or allowable, reduces your cost basis in the property that you report when you sell or otherwise dispose of the property.
The gift tax applies to the transfer by gift of any type of property. You make a gift if you give property (including money), or the use of or income from property, without expecting to receive something of at least equal value in return. If you sell something at less than its full value or if you make an interest-free or reduced-interest loan, you may be making a gift. For additional information, review Form 709 and its instructions.
See this IRS article regarding the gift recipient's basis in the property.
You may want to obtain professional advice before proceeding. This will impact your estate planning as well as your/your son's income tax returns.
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