A Guy Called Tyketto wrote:

Depending on if he wants to go through insert Class B airspace
here, He could use I70 all the way through, then I15 down.

Do NOT follow I-70 west of Denver. Unless, of course, you're in a
Pilatus or a twin with ceiling of 18K. About 60 miles west of Denver,
on I-70, is the Eisenhower Tunnel. The tunnel itself is at 11K, and
the mountains it runs thru (the Continental Divide and Loveland Ski
area) are 13K. And there's no way to turn around.

Feel free to follow I-70 to Denver, then turn either north or south and
follow I-25 to get around the mountains.

But
depending on the starting point, I40 would be great. Even better, make
it a really fun trip and follow old Rte. 66, which should take him
through smaller fields, and avoid most to all Class B (Chicago
excepting) airspace, all the way to IGM, then track the BLD R-108
inbound, and set.

Someday, I'm gonna do the Rte 66 trip. Someday.

*sigh*