You don't say if you need to keep solely to an ACA route but I would guess not nor do you say how you are doing services (Food & Lodging).
Anyway, if I were attempting to do what he is doing, I would take the TransAm to near Ordway, CO then take CO-10 to Walsenburg. Then US-160 to AZ-98 in Arizona. Then US-89 in Page, AZ, to Kanab. Then US-89A to AZ-389 St. George, UT. Then up to Enterprise, UT and rejoin the Western Express to the end.
Remember that by the time he reaches Colorado, he should be strong enough to do any climb, especially unloaded. However, he may (like me) have lung issues so elevation does factor in. You should understand that having a day or two that has a big climb but with much less mileage does not add any days. In other words, typically, taking a 200 mile route with a couple of big climbs is quicker than taking a 300 flatter route. If he has lung issues, he can angle down toward US-64 from Walsenburg then take it through northern New Mexico. over to where it intersects with the above mentioned US-160. This is lower but not nearly as scenic and has more traffic.
If you have extra days available, the ride from Mexican Hat, UT, to Kayenta, AZ, is wonderful through Monument Valley and so is the western Express through Utah.
Best, John