why he doesn't keep his head straight when I take him to get burp he just move backside

A. Maybe you do not have an understanding of this thing but kids don’t have stability of the head for first six months after completing the time of three months it would be partial stable and after six months total control prior to that if you don’t give support to the head and neck of the child while lifting the child their head is going to bend backwards there is no strength in the neck and head of the child