i am not aware of any medical condition that would cause his hair to immediately change from black to reddish-brown in this short period of time. it may be wise to have him medically-evaluated however.
what can cause a 10 year old boy's head of hair to change from black to reddish brown in a matter of hours? . my 10 year old was playing in the back yard but then started having a slight headache. when he came inside i notice his hair on one side was changing in color from black to light brown i also notice he had what looked like 2 mosquito bites on his arms. he had no temperature but was feeling a bit fatigued. but we went on to basketball practice none the less. by the end of the evening his entire head of hair was now the color reddish brown.