There are many types of skin cancer. The type that looks like a mole is malignant melanoma and it is fatal. Contrary to what most people believe, it isn't "just skin cancer". It can spread to internal organs and kill you.
30% of melanomas start out in existing moles. 70% grow out of normal skin. It's ok that your boyfriend has lots of moles, as long as none of them are changing or growing bigger. The best thing to do is to have him do monthly self checks. Get naked and look at all the moles. If he has tons, too many to remember what they look like and if they've changed, then take pictures. If one of his moles is changing and growing, then go see a doctor and have it biopsied. If a new mole pops up that is also growing fast, then go get it biopsied.
Changes in moles that should be checked by a doctor can be remembered by the ABCD Rule.
A = Asymetry - is the mole symetrical (both sides same shape) or not? Melanomas often are not symetrical
B = Border - does the mole have a clean, sharp border, or does it look jagged and fuzzy? Melanomas often have fuzzy or jagged borders, pigment appears to leak into surrounding skin.
C - Colour - is the mole one uniform colour, or does it have several shades and/or colours? Melanomas often have several shades of brown, black, red, blue, pink, or white.
D - Diameter - is the mole less than 6 mm (size of a pencil eraser) in diameter? Melanomas are fast growing and will typically grow bigger than 6 mm.
If your boyfriend's moles have one or more of the ABCD warning signs, get them checked out by a doctor. A good website for more info on melanoma is www.mpip.org