What lack of vitamins could make the hair fall ?? for a young guy

Sorry, not vitamins: There are no vitamins that will keep hair from falling out. Loss of hair in a man is most commonly genetic. Hair loss in the front and on top of the head is referred to as male patterned baldness. If older men in your family have male patterned baldness then you are probably just beginning at a younger age than usual. Rogaine, (minoxidil) sold OTC, could be helpful.

Vitamin D3 & Biotin: Hair falls out for many reasons. One common reason is telogen effluvium-stress-induced hair loss. Often the stress that triggers hair loss occurred months before hair begins to thin. Vitamin D has been implicated in hair loss for many people. In my experience, the majority of people (especially people of color in northern climates) are Vitamin D deficient. Biotin is critical for hair growth.