The dark mark on your son's face is simply precipitated silver. What happens is that silver nitrate reacts with and precipitates on (or under) the skin. Subsequently, the silver precipitate darkens when exposed to sunlight, just like photographic film. The mark that appears is initially white, then turns brown, and gradually darkens further. It is not possible to wash off the stain or remove it in any other way. However, there is no need for you to worry the mark is neither harmful nor permanent. Silver nitrate stains on skin disappear on their own. As your son's body goes through the natural process of discarding dead skin cells and growing new skin, the mark will gradually disappear.
Should your son ever have trouble with a wart again, you might want to try other home remedies that are as effective - try applying castor oil or cashew nut oil on the affected area regularly for at least a month. You can also try rubbing warts with cut onions or potatoes, several times each day. Another treatment that is reputed to be effective is the external application of the juice of papaya, pineapple, or fig. A fruit-heavy diet also helps, including periodic all-fruit diets for a day or two.
answered by G M