as someone else stated, it does take some time to adjust sleeping habits while the vehicle is in motion. it does become one of trust too. the other driver can be the utmost safest driver in the world, but your hubby doesn't know this. also, even the very best team driver will hit a bump or two in the road, causing vibrations that even the very best sleeper cabs to feel.
my only advice here would be for your hubby to NOT drink any caffinated beverages 2 to 4 hours before bedtime. if he feels the "need" for a coffee or other caffinated beverage, tell him "caffine free" is the way to go. also, have him cut down on his sugar intake as well, as sugar will provide short bursts of energy that can possibly interfere with his sleep time.