u/Just_Highlight_9299

… one of the Commander Winslow sons.

Winslow and his wife had 6 children, of which 3 were boys. They lived near Washington, which would be close to the place where the Testament take place.
His “father” was respected Commander and that could be the one they were mentioning in the car.
It would be such an interesting plot twist if he were to find out Agnes mother killed his “father”.