My baby has a redish mark on the back of his head since birth, now 3months old. should i be worried, what can i use?

Birthmark?: Please ask his doctor if that is a birthmark.

Likely birthmark: Sounds like a birthmark, one of the most common. People call these "stork bites"; medically they are capillary hemangiomas and are not dangerous at all. They can last for years; they usually disappear over time, but many adults still have them. Show your baby's doctor at the next visit and/or send them a photo/do a telemed visit and they'll be able to tell you for sure.