It is normal for a 13 year old boy to have sweaty underarm hair because 13 year old boys have usually entered puberty.
13 year old boy with sweaty armpit hair is one of the normal physiological phenomena of puberty, the pubertal development of boys will appear some changes, such as there will be a sharp increase in weight and height, the emergence of beard, underarm sweaty hair and other changes.
The growth rate of the child in this period is very fast, you need to give the child more food rich in high quality protein, but also let the child eat more fruits and vegetables, do not let the child excessive picky eating, so as not to cause damage to the child’s growth.
Boys puberty some children on the earlier side, some children on the later side, there are individual differences, do not have to worry too much.