My son is 3months, his polydactyly finger is swollen and black please assist?

Your post lacks important data. Polydactyly can be as pronounced as a full normal extra digit or a thin dangling nubbin with minimal form/tissue. The minor ones often self destruct/self amputate when their blood supply does not adequately supply its needs. If you are worried about it contact his doc. When posting here the more data you provide the more we can help.

Go immediately to an emergency room, not an urgent care center. A black finger could be a sign of gangrene or at least a severe injury. The people in the emergency room will be able to help you and your son.