Marathon Nursing Sessions - any advice?

In the last day or so, Charlie's nursi sessions have grown longer and longer. He is three weeks old today, and he had been getting progressively more efficient at nursing (his sucks were stronger, latch was better, total time shorter). Now for some reason, he wants to nurse upwards of an hour and still seems hungry when I make his stop. He starts out great, but quickly gets sleepy and starts to "nibble" instead of really drinking well. I have to constantly try to keep him awake or take him off the breast to get him eating well again. If I do take him off, he acts like he is completely starving and it is hard to get him relatched well because he is flailing his arms and tries to start sucking before I can get him a really good mouth full of my breast. This has started to cause soreness on one side which makes me want to nurse for an hour or more even less! I have a two year old to take care of, so I can't spend one of every two hours feeding Charlie, plus at night it is worse. Last night, in an hour and forty minutes (100 minutes) we spent 70 minutes nursing. Then I had to rock him and try to pacify him to get him back to sleep. It was almost time to start again by the time we were finished!

Anybody have any advice for me?