Did he have the numbness right after the injury, or has it developed in the past couple of days?
If the numbness has developed in the past couple of days, after the cast was put on, it is most likely from swelling in the cast, putting pressure on the nerves.
When you sit on your foot and it goes to sleep, that is from compressing the nerves. When you relieve the pressure, the foot wakes up. So, pressure from swelling inside the cast is the same thing and can cause the toes to go to sleep.
If this is the case, he should elevate the foot above the heart level. That requires him to be flat on his back on the couch, with the foot up on the back of the couch. Or, lying flat in bed, with the foot way up on pillows, above the heart. The elevation helps to get the edema, blood, and swelling back up the leg and into the body.
He should wriggles his toes, several times an hour. This helps to pump the edema and swelling back into the body. The veins and lymphatics are low pressure systems, that have to have the contraction of muscles, to help pump the blood and fluids back to the body.
If the elevation and wriggling of the toes does not make the numbness decrease pretty quickly, you need to contact his orthopedic surgeon for advice. Sometimes, the cast has to be bivalved (cut on the sides and spread apart a little), to make more room inside.
The nerves are very sensitive, and show symptoms first. But, if there is enough pressure inside the cast, it can affect the circulation (blood supply) also. So, again, if the elevation doesn't help pretty fast, contact the surgeon.
However, if he has had the numbness from the time of the injury, then it may be that one (or more) of the peripheral nerves has been injured. It is not uncommon for the nerves to be stretched at the time of injury, when there is no stability in the leg. Nerves are very picky, and do not tolerate injury well at all. Most of the time, the injury is neuropraxia, which means the nerve is okay and it will wake up in a little while.
In this case, the surgeon should have documented that the numbness was present in your son's medical records. This type of numbness has to be watched very closely. Numbness which affects the sole of the foot in tibial fractures can be a significant problem. Patients who cannot feel the sole of the foot are susceptible to developing chronic pressure ulcers and infections (like a diabetic). In open fractures, if the nerve has been torn in half, some times the treatment is a primary amputation. So, you can see, that this can be a significant problem. Usually in closed fractures, the nerve is still in continuity (connected), so patients are given a trial of life to see if the nerve wakes up on its own (before amputation would be ever considered).
So, numbness from the injury is usually picked up by the surgeon at the time of the first evaluation. However, not always. So, if the numbness has been there since the beginning, and the surgeon didn't talk to you about it, then you need to contact him/her.
Thus, again, if the numbness has just developed, try elevation and wriggling of the toes. If this takes care of the problem, great. He will just have to keep the foot elevated more during these first few days.
But, if the numbness does not get better pretty quickly (4 to 6 hours), you need to contact the surgeon.
If the numbness has been there since the beginning, and the surgeon did not talk to you about it, you need to contact the surgeon.
Good luck. Hope your son feels better really soon.