Duragesic is quite powerful stuff.  It is also known as fentanyl, which is
used as an anaesthetic.  It is a MU agonist, and pretty much is many many
times more potent than Morphine.

There could be withdrawal symptoms, which is the case with any Opiate.  They
are not usually life threatening, and if under a doctors care, he could
eventually be weened off with little problem most likely.

As for getting sick, it may subside after he gains tolerance.  However, I
will tell you, keep the patch off of heating blankets, out of the sun, and
do not apply it to the side he lays on when he sleeps.  Increasing the heat
on this patch increases the permeability of the membrane in which the gel
passes through, resulting in a higher dose.

I am NOT a pharmacist or medical professional.  Do not take what I say as
fact without consulting your doctor or pharmacist.