apologies for thread drift, here is a suggestion, take the suggestion of don meixner and put all the thumb on the off beat as an exercise, so the player starts with a finger then plays the thumb.
if a player gets used to this, it can vary the sound,so the the thumb can occasionally throw in a thumb on the off beat, slightly like bill broonzy. if a player plays five string banjo up picking with fingers and drop thumbing, it is a very similiar technique.