Change status from F1 to F3 and back

As you sponsored your son under the F-1 category as the unmarried son of a U.S. citizen and his status automatically changed to F-3 as the married son of a U.S. citizen, his subsequent divorce allowed him to return to the F-1 category without loss of priority date. That is because there has never been a break in his eligibility to immigrate. Opposed to your son's situation are those in which green card holder parents sponsor their unmarried sons or daughters over the age of 21 under the F-2B category for unmarried sons or daughters of permanent residents who during the process marry and then divorce with the parent remaining a green card holder during this time. In the latter example, there is a break since there is no visa category sponsorship possible for a married son or daughter of a permanent resident and therefore in that case as opposed to yours, the priority date is lost.