Unless he is VERY comforable with and familiar with towing in steep mountainous terrain with narrow highways, no shoulders, and no guard rails, do not go north from Durango. US 550 between Durango and Ouray is not for the faint of heart.
Best go east on US 160. He will still have to climb Wolf Creek Pass which is also steep climb and high pass, but a much better highway. Two lanes all the way up, places to pull off if needed to cool down, guard rails, and only one real curve of any consequence. Simplest route is US 160 all the way to I-25 at Walsenburg and then north. Other options are north from Alamosa, but more changing of highways.
If he has an aversion to steep climbs, there are other longer options.