Google suggests 2650 miles on a fairly direct route to southern Vermont. "Well after Labor Day" start date in El Paso, Texas to catch some of "Vermont in fall". Maybe five weeks? 76 miles (122 km) a day, seven days a week, no rest or weather days.
Shrug. It's Norts' call.
I think must have missed the “well after” part. My thought would be Amtrak to CHI. ACA has a route east to NYC. Maybe use part of that and its Northern Tier route to the Erie Canal towpath. That would get him a lot of the way. Potentially nasty weather would be a big concern if mine. I remember camping along Lake Erie a bit west of Buffalo while crossing the country. It was mid-August. We had a fire to keep warm. Really cold a wet day east of Rochester, NY. Fortunately, we got hosted that night. Those lakes can cause unexpected conditions. Booneville, NY was very cold for August. You’d definitely want to stay as far south as possible but stay north of the Berkshires.