You both are adults and ultimately it is going to be your life. However, it is always helpful to convince the family as much as possible.
1) Make sure your boyfriend definitely wants to go ahead with this marriage.
2) Explore - what is it that his parents are against about this marriage. Be bold and talk to them along with your boyfriend, listen to their side of the story.
3) If they have reasonable concerns about this marriage - try or work in resolving those.
Hopefully through these steps you might get some help.