How can get my boyfriend to gentler when we have ?

im 16 and my boyfriend is quite alot older than me and when we have he's really rough with me and its sometimes painfull cos he's So Big (his thing is like 9 inch's or something) and like when we do it he sometimes like putting me against a wall and taking me from behind but he's so rough ive ped out twice what can i do? How can i make him be more gentle