I my view this is not a medical condition, but an habit which may be there with your husband for long time. As per the descrption, when there is more use of the mouth, than there is more saliva formation and it is the normal process as the oral cavity has to be kept moist to make the movement inside the oral cavity smooth.
If there is some oral condition which is associated with this excessive salivation
, than that oral condition can be the reason for this problem, but as you have described that there is nothing abnormal with the oral health.
Even though I would suggest you to consult a dentist
regarding this problem and if something found to be associated than have treatment for that.
Thanks and take care.