I have bruises.

Okay, I have been dating a boy for 6 months & he's my Bestfriend but sometimes we argue & when we do I just want to walk away from him but when I do he gets kinda mean... He grabs me & throws my on his bed & then he will pull me to him. I have really bad bruises on my arms but they don't hurt. I told him about them & he cried & said it'll never happen again. Idk what to do??? HELP.