You can use an eyebrow pencil which is darker than your natural colour. You could also dye them or get them tinted at some kind of salon.
I don't know, we girls usually fill them out with eyeshadow or some type of eyebrow makeup thing, but that would most likely look like crap on a guy... =/ I doubt mascara would lead to very satisfactory results, either. Perhaps go see one of the ladies at the drugstore makeup counter? Maybe they'd have an idea.
There are eyebrow pencils that fill out the brow a bit. If you try one, look on you tube for application videos. You can also try putting an oil mix of 2/3 olive oil and 1/3 castor oil on. It moisturizes and promotes hair growth.
