My 7 year old son has been running a fever for 6 days. Ihave taken him to his pediatrician twice and he is beingtreated for a sinus infection. Should he be seen by another doctor?

Probably. If the fever is continuing even after being treated your pediatrician would want to know. At the very least call and ask for advice or just make an appointment now.
ALWAYS FOLLOW UP. Until the day comes when we may be able to clone ourselves, remember we can't go home with you. Your regular , consistent feedback is extremely important.
Other Problems. If he has had a sustained high fever for 6 days it might be something as simple as the flu that will eventually get better on its own. However there are other more serious conditions that need to be considered such as kawasaki's disease or other conditions associated with high fevers. He needs to be seen again.
Probably. Most fevers in children last 3-5 days. If the infection is bacterial, antibiotics usually help within 48 hrs. If your son has persistent fever over 101 for more than 48 hrs after his last visit, you should call. The doctor may want to reevaluate him or do more testing.