This is a great thread BTW!
Ennis had a very good idea of the nature of the relationship between Jack and his father - he never taught him a thing, never went to see him ride, i think Jack sought validation from his father, following in his footsteps to some degree, but it was never forthcoming.
Knowing this i think Ennis was in some way validating Jack to the old man. He was a man he had been proud to know, regardless of the relationship between them. Ennis wanted Jack's dad to know his son wasn't just a waste of space, that's why he made it plain he would perform this final task with pride. Jack had been worth it.
Also maybe it was a sense of Ennis getting one over on Jack's dad. Dad wasn't interested and look what he missed out on - a relationship with a son other men would be proud of knowing, and now it's too late to start trying. I love Ennis for this now!