My son broke his humerus bone on wednesday and they put a new cast on it yesterday and it feels like its moving around. is that normal?

There may be: some "motion" but as long as its not Painful (t this stage untreated fractures are very painful) its probably only the dressing underneath allowing some motion! (a cast which is too "tight" is VERY harmful! If this is really concerning you go back to the place/doctors where he was treated. Hope this helps! Dr Z

Swelling dissipates: The acute swelling from the initial injury subsides over the first week or so. The cast would, therefore, necessarily become loose feeling during the reduction in arm swelling. Contact your treating physician's office to determine if a new cast is now indicated.

Be seen : No, not normal but not unusual. The danger in putting a cast on too soon after an injury is that the arms swelling can increase or decrease, but the cast does not change size. So, it sounds like the swelling in your sons case has gone down and the cast is not fitting well. He would be well served to go back and have the cast changed so it is functioning to protect the fracture as it should.