Is separate DS -160 form needed for my baby for US B1/B2 visa? and should I pay a separate fee for him(= 160 US$)?
Yes. A separate DS-160 must be filed for each person applying for a visa, including infants. See, for example, this page: "[A]ll children need a visa for travel to the U.S." And you will need to pay the applicable visa fee for the child's nationality and visa type just like any other applicant.
Check the details on the website of your local consulate, but children 13 and under generally do not need to appear for the visa interview.
The US Department of State has some tips on taking photos of young children for this purpose.