It does seem harsh for a first time offender. Could there have possibly been previous offenses that you are not aware of? Either way though prison is not the best situation, there are programs available that he can take advantage of including drug classes, furthering his education, or perhaps even learning a marketable trade. He gets to choose whether he just wants to be warehoused, or actually does something with his time.
I would send the card, but be aware that this may lead him to believe that you will also be willing to put money on his books and advocate on his behalf.