Touring Bhutan is a challenge for a traveller with physical disabilities, but you'll have a guide, driver and vehicle at your disposal, so this is possible with some planning. The Bhutanese are eager to help, and agencies should be able to arrange a strong companion to assist with moving about and getting in and out of vehicles.
Mobility impaired travellers will find that roads are rough and pavements, where they exist, often have holes and sometimes steps. Hotels and public buildings rarely have wheelchair access or lifts, and only a few hotels at the top end have bathrooms designed to accommodate wheelchairs.
Download Lonely Planet's free Accessible Travel guides from http://lptravel.to/AccessibleTravel.