Hoarseness is usually caused by a problem in the vocal cords. Most cases of hoarseness are associated with inflammation of the larynx (laryngitis).
Obviously the cause of his laryngitis was surgery, with the tubes being placed down his throat. It is possible that he over-used his vocal cords too soon and caused a set-back. If the delicate tissue had begun to heal and they were stressed it could have resulted in the secondary hoarseness.
But persistent hoarseness (hoarseness that lingers for weeks or months) can be caused by a variety of problems ranging from trivial to dangerous. Since his has lasted for more than two weeks he should probably make an appointment with his doctor.
It is possible that the new hoarseness he is experiencing is unrelated to the trauma experienced during surgery, so it is best to have things checked out.
When should he see an otolaryngologist (ENT doctor)?
Tips to prevent/treat hoarseness at home:
If he smokes he needs to quit
Avoid products like alcohol and caffeine that dehydrate the body.
Drink plenty of water and humify the home.
Avoid spicy foods.
I hope this helps you!