My 4 months old baby stool color is grey. What are the reasons for grey color of stool and treatments. He is bottel feeding?

Usually transient. Stool color changes are usually transient and not serious. Exceptions are: red to black stool; persistent chalk-colored stool.
?normal. This could be normal. White stools could signify liver disease. I would observe for a few days. Check with your dr. If persists.