Some people care, some don't. To make fun, tease, or joke about someone's body parts can harm ones self esteem. Refraining from it helps you and others feel body positive. An apology would certainly be nice. If your brother worries about his thin hair then maybe you can encourage him to talk to his doctor about his options, such as a prescription, an OTC treatment, or a sensitive hair cleansing regimen. If you wanted to buy him something to prevent his thinning, hair loss then try sites like
Natural Medicines Comprehensive Database which list medicines for conditions like alopecia.