Your son living there has no bearing on this. If the legal property owner lived in the residence as their primary residence for at least 2 of the last 5 years they owned the property, then the legal property owner can exempt up to $250K of the gain from taxes when they sell. ($500K if married filing joint, and the spouse meets the "2 of last 5 years" requirement also.)
Note that for the "2 of last 5" rule, that breaks down to it having been your primary residence for at least 730 days of the last 1826 days you owned the property, counting back from the closing date of the sale.
