What could cause a 7 yr old to have significant amounts of hair fall out every time it is brushed?

Many causes: It is possible that he may have seborrhoic dermatitis which could cause his hair to fall out. Other possibility is hypothyroidism or may just be a normal phenomenon.

Many things: Thanks for your question, many things including nutritional defiencies, hair products, health conditions like thyroid disease, and genetics could be contributing to hair loss. Please see your pediatrician asap.