You may write him a letter or call at his home.Perhaps you should give him a phone call and ask him what you have done wrong.If it is really your fault,you should apologize to him.（I hope you two will make up as soon as possible and I believe that he will become your good friend agian.)
第一个what wrong you have done有语法错误,第二个ask后面如果加从句后可加that或是wh－的词,但是不能同时加.两句的前后都重复了wrong,而在英语里一般会避免重复.
括号里是我自己乱加的,不知道是不是符合题意.