Any concern you have should be addressed. There are problems that can be caused by having a cast on that's too tight. Acute Compartment Syndrome (ACS) is the most serious one. Rising pressure between the layers of bone, muscle, and fascia can cut off the blood supply to the leg.
Call your doctor and report any unusual symptoms. Pay attention to the temperature and color of the skin of the leg above the cast and in the foot or toes below the cast. They should be warm and pink or flesh-colored. Blue, black, or white are signs of blood loss.
Is your son complaining of numbness, tingling, or pain? Again, these are other symptoms of the cast being too tight.
Serious problems can be avoided by early detection and treatment. Trust your intuition and common sense.