Boys in My Therapy Book

Why do teenage boys stop caring about their hygiene out of the blue. My son is almost 13 and all of a sudden he has stopped giving a shit about his personal appearance and grooming habits. It seems like overnight he went from a prim and proper little man wanting to wear button down shirts and khaki to baggy sweat pants and god forbid he wore his mothers shirt yesterday. Not out of any gender identity crisis but because it happened to be within reach. I don’t know what to do with him. He is such a good looking young boy and then he walks out of his bedroom with cloth that clearly came off the floor or out of the hamper, hair and teeth un brushed and ratty ass flip flops on. (I am about to throw those away) . How do you teach a 13 year old boy to care about how he looks? UGH.