My husbands hair is falling out a lot all of sudden. he's only 19. no bald patches but hair is thinning.

Stress?: Is he under unusual amounts of stress? Otherwise could be normal male pattern balding. Some men start very early. Does it run in his family? Other possibilities are hormone irregularities and he would need blood tests to check that

Thinning hair: Please ask him to see his physician for a history, physical and screening bloodwork. Some early male pattern balding is genetically early, but 19 is very young.