They need to know the adjusted basis of the gift for tax purposes when the donee eventually sells the property.
They also need to know the fair market value of the property to determine the amount of the gift.
You had a part gift/part sale; you sold the property for less than it's fair market value to a relative.
Your son paid $386,000 when the home had a FMV of $455,000 so the amount of the gift was ~$69,000 and the basis would be $386,000 (which is the greater of your adjusted basis and what your son paid).
Note that if you are married, you could give your son up to $36,000 in 2024 without even be required to a file a gift tax return. Also, if your son is married that figure would double to $72,000 ($36k to your son and $36k to his spouse).
