My son is 17 months old. he was born with mongolian spots. when do they go away?

Mongolian spots: They are normal accumulations of melanocytes (the cells which give your skin color). Over time they may spread out over the skin surface and thus lighten but in general that is his pigment. They are usually symmetrically distributed from the spinal column as melanocytes are neural crest derivatives.

Maybe never: These are common patches of skin where the pigment producing cells are in greater abundance and activity.The patch may lighten some over time but may also be there forever.