I-80 to I-25 to I-10?
That's probably the best way if he wants an all-freeway route, but it's not a very direct route.
Google says it's 1170 miles, 17 hours and 7 min. Depending what time he hits Denver and the Springs, it could take quite a bit longer.
Mapquest says 1253 miles, 18 hrs. 49 min. That's quite a discrepancy, but I suspect MQ is more accurate with respect to mileage.
AAA says 1170 miles, 16 hrs. 42 min.
My first thought was:
I-80 west to York.
US 81 south to Salina.
I-135 south to McPherson.
K-61 southwest through Hutch to Pratt.
US 54 the rest of the way.
Google says this is 1053 miles, 16 hours and 35 min.
Mapquest says 1029 miles, 17 hrs. 4 min.
AAA says 1021 miles, 16 hours.
That's essentially the route that all three online services suggest, except between Salina and Minneola:
From Salina, I-70 west to the Ellsworth exit.
K-156 southwest to Great Bend.
US 56 southwest to Dodge City.
US 283 south to Minneola.
US 54 the rest of the way.
According to Google, this is 1008 miles, 16 hours 20 min.
Mapquest says 1017 miles, 16 hrs. 53 min.
AAA says 1008 miles, 16 hours.
So, bottom line:
Google says the US 54 route is 170 miles shorter than the interstates, and saves about an hour.
Mapquest suggests the same US 54 route, but they claim it's 236 miles shorter, and saves about 2 hours.
AAA says it's 162 miles shorter, and saves about an hour.
So that's what I'd do. But it might depend whether your dad is ok with 2-lane highways.