Why don't you try talking to him? Tell him how you felt, sympathize with how he feels, say that he's not a dumbass, and sincerely apologize. Then swear to not to call him names again.
I GOT INTO A FIGHT WITH MY BEST FRIEND AND CALLED HIM A DUMB*SS OUT OF ANGER. HE JUST WENT SILENT, SAID HE HAD TO GO, AND LEFT. NOW I FEEL BAD BECAUSE I KNOW HE LEFT BECAUSE I MADE HIM FEEL BAD. I DON'T KNOW WHAT TO DO, I LOST ALL MOTIVATION TO DO STUFF. I TRIED ASKING ONE OF MY OTHER FRIENDS FOR HELP, BUT HE WASN'T MUCH HELP AT ALL. MAYBE I'LL JUST WATCH EDDSWORLD OR SOMETHING...