Once you hit Montana you're pretty much good. There actually is a speed limit there now - didn't used to be, which is why I ended up hitting I90 at...uh...55 mph, Your Honor. I swear. Take care at Fourth of July Pass (if you're going that way) and stay safe when you hit the Promised Land. :-)
Worked for me driving tractor-trailer. Might consider some of the Kansas roads if I was hoping to avoid scales, though. But then that State has roving truck inspectors who will pull a truck over on the side of the road and use portable scales, and a mechanic's creeper to roll under the rig and give it a level One inspection.
Other States use weigh Stations and Inspection barns, if they inspect much at all. Colorado wants to see one's fuel tax agreement (IFTA), or pay a fee to cross the State. Wants the first six numbers of the VIN on the side where they can read it too, or they'll wave you over to come inside and show them paperwork...and maybe get inspected.
New Mexico is similar, except they want the New Mexico specific agreement. First they ask for the IFTA, then when you show them that, they get real rude saying they "don't care about that" then ask for the New Mexico paper. F 'em. I hate that State.
Unless he’s taking a very long way home he will not be going through Kansas, and nowhere near Denver for sure. 80 goes through Nebraska, and 90 is even faster and a lot prettier once you get to Wisconsin.