We used to soak old stiff dock lines in water with fabric softener overnight and that helped. I would think that would work on his Carharts too.
I washed and wore my Carharts until they broke in from use when I worked in them. I have a parka made by them well worn now and showing it's age I still use at home but my wife forbids me to wear in public. I have a new one hanging in the garage she bought me but it isn't the same yet as that old one, maybe one day I'll break it in too.