My 2nd younger brother who is 15 now has a line of hair extending from his belly button to his pubic hair, is his puberty done? is he done growing in height and other areas?

A better gauge of final pubertal growth is their height. When their increasing height slows to 1 inch or less/yr they are nearing the end. An x-ray of the wrist/hand can reveal the growth plates. If they are open-more height is possible, closed- their height is fixed. There are other aspects of growth that continue into the 20's. Brain basic connections continue in most until 25.