For those who don't know, Preimplantation Genetic Testing (PGT) has been used for years to screen for genetic problems when parents are known to have a particular genetic abnormality, when parents want to avoid an X-linked genetic condition by avoiding a male child, or when a mother (e.g., an older one) is at high risk of having a child with a genetic disorder. PGT is used in conjunction with in vitro fertilization (IVF) and avoids the difficult decision about terminating a pregnancy when it's determined that a child would have the targeted genetic disease. Still, PGT is not without controversy.
PGT for HLA matching (PGT/HLA) is an extension of that process that can detect the presence of a genetic disorder as well as a potential stem cell donor for a sibling, where the newborn's umbilical cord blood could be used to save the sibling's life.
This article by Dayal and Athanasiadis is one of the better summaries I've seen of the PGT process from the medical point of view, including PGT/HLA.
This article by Devolder, while not recent, covers the controversy over the process while
this article by de Wert, Liebaers, and Van de Velde discusses the ethics of PGT/HLA for conceiving a child with a certain HLA type and of PGT/HLA used to obtain stem cells directly (an area of research).
In considering PGT/HLA you'll first want to discuss the medical case for this approach with your son's doctors and of course you'd confirm whether an unrelated matched donor is already available. (I know your older son has already been ruled out.) If PGT/HLA is found to be a medical choice, then you, your family, and genetic counselors will have to discuss the other issues. It's a very personal decision, often influenced by religious beliefs.
I don't know a family that has used PGT for HLA matching but I know there is a growing number of them.