It sounds like your boyfriend has what is called a congenital melanocytic nevus. These are darkly colored spots that occur on the skin, usually present from birth. Most congenital melanocytic nevi contain thick, dark hairs such as the ones you describe. The exact cause is currently unknown, but they likely represent a disruption of the normal migration of some pigment containing skin cells (melanocytes) during fetal development.
Most congenital melanocytic nevi do not require treatment, especially if they are in an area like the arm, where they are not likely to be a big cosmetic problem. There is a small risk that a congenital melanocytic nevus could give rise to a melanoma, which is a serious form of skin cancer; however, this risk is very low with a small nevus of the size your boyfriend has. Nevertheless, he should make certain to keep an eye on it and alert his dermatologist
or primary care doctor
if he notices any changes in its size, color, or texture.
If the spot is a cosmetic problem, sometimes they can be surgically removed, although scar tissue afterwards may be an issue. There is some evidence that laser-based 'bleaching' can be used to lighten the areas, but this is controversial and not routinely done by all dermatologists. If you boyfriend is interested in removal or treatment, he should discuss this with his dermatologist.