I would made him understand what are the ill - effects of mobile phone . I would also tell him that by keeping all concentration on phone it will result in weakening of ciliary muscles  and straining of eyes  and can cause irritation while reading or looking.

After that , I will tell him to spent time with his friend so that the relationship or bonding between our friends remain constant. If does'nt do so ,at last I would like to complain his parents so that his parents could take some strict steps and also made him understand.