It was called "The Guns Of Will Sonnett."
It ran for two years only from 1967-1969 and
starred veteran movie and TV star Walter Brennan and
newcomer Dack Rambo. It was one of Aaron Spelling's
first shows ever, which he did in association with
Danny Thomas.
It was never out on VHS, ever. I never saw it anywhere on
syndicated TV. I had not even thought about
it since the 60's until I ran into a new cheaply put together
DVD set of the show.
I have now watched all 50 episodes, including the last
one https://www.youtube.com/watch?v=62kzX-5v_LE
which wrapped it all up with a finale (very unusual for a
60's show to do). Somebody has just posted some episodes

The story of a young man who was raised by his grandfather
since birth and decides to go find his father is definitely
one of the best westerns ever on TV which most people
don't know about. It pretty much disappeared after 1970.
The show gives you a strong feeling for the traditional
Movie/TV "Old West" set in post-Civil War 1870's.
Storylines are intriguing, a bit too rushed for a 30 minute
show, acting is superb and it is amazing to see the then
unknown talent that popped up in the show.
Jack Nicholson, Charles Grodin, Kevin Hagen,
Ellen Corby,
Strother Martin, Dennis Hopper and others including Jesse Pearson who starred in "Bye Bye Birdie."
The DVD set is copied from some edited version put together from
some TV syndication version used somewhere. As a
result, the color is faded and there are clearly a few minutes
missing from each episode. At least 3 times, the editing
makes no sense and you know something was cut out.
The director must have had a friend, because the same
bartender pops up in different towns in different states
many times.

Opening Card on Season 1
when ABC was promoting
TV shows now in Color.
It was really enjoyable seeing this lost gem after
so many years. No Brag, Just Fact.
We searched for a man named Jim Sonnett.
And the legends folks tell may be true.
Most call him gunman and killer.
He’s my son, who I hardly knew.
I raised Jim’s boy from the cradle,
'Till the day he said to me,
"I have to go find my father."
And I reckon that’s how it should be.
So we ride, Jim’s boy and me.