So, it's entirely possible that the rogue code remains unactivated in his cells but remain rather ineffective because the white cells come from a different lineage due to the transplant.

Funnily enough Retroviruses are also the major vector in gene therapy.

Retroviruses incorporate into the host genome and is important for gene therapy. This allows for a long lasting effect and is essential depending on cell type. Different cell types have different turn-around rates. If the gene of interest is for a protein, you need incorporated DNA for continual expression.