3 y.o.  thumb sucking

I would like to learn how I can help with my grandson's finger(s) sucking.  I know this is fairly normal behavior but it can affect his teeth formation and he is more apt to get more "germs". He is a bright, happy little boy who's parents work long hours 9am-7pm (son), 9am-6pm (daughter in law).  The 3 y.o.boy and 8 mo. old girl both attend a high quality day school/care but get picked up and are home at about 8pm at which time both parents get home plus or minus a half hour from 8pm.  The parents cook or eat take out dinners, have some play time with both little ones, then bath and bedtime from 8pm-9:30pm but have the TV on sports or appropriate favorite shows.

I have a background as a psychiatric nurse with a MA in Human Behavior, BS in nursing with minor in psychology and am aware of psychological issues.  I like to be very careful not to interfere/intrude in the lives of my son and daughter in law and currently have a very congenial relationship as well as with the parents of my daughter in law who are loving grandparents.  Personally, I feel the little boy is still adjusting to the arrival of his baby sister plus the fact that there is little time for 1 to 1 daily attention since they live busy lives.  The parents are both very intelligent and have responsible jobs that they enjoy (and equally enjoy their children).

How can I help besides the nurturing I do when I visit them once midweek in the evening and a few hours on the weekend, a privelege I am very fortunate to have.  The boy has irritable, sometimes demanding moments but generally is self directed and happy. Fortunately, my son and his wife do not make an issue of his finger sucking and deal well with his tantrums which in part happens when he is tired.  They both are loving parents but I can see how tired they are from working and parenthood with the long hours they have in their working week.  

Help!  Thanking you in advance.