Expansion joints at bridges are a problem for everyone that tows anything. His situation is not unique. We all experience that! And being as heavy as he is, he's going to feel it more. You can soften the suspension all you want, but the tires are still going to take the same "hit" no matter what rides on them. 
This is another reason why I avoid interstate at all costs, as much as possible. Faster speeds, harder hits on the tires! All the bells and whistles in the world to make it more comfortable for the passenger won't change the blunt the tires takes.