Thank you for your query
Compression on the arteries, veins or nerves supplying the arm and forearm may lead to such symptoms. Similar changes in color of the extremities in response to cold
is called Raynaud's phenomenon. The pressure is most likely at the armpit or axilla
level and that is why such changes occur with the movement of arms. Your son needs a thorough evaluation in terms of Doppler of the arm, nerve conduction studies etc. It might not turn out to be anything significant and that is what we hope for. But it may also point to something serious like a mass pressing in the axilla. You should consult a pediatric surgeon for the same.
Hope that helps
God bless you