might you try reinforcing that which is positive elsewise in his life? What is your relationship like with his father? I also wonder does his father treat his custodial time as a holiday? Or is it treated as though his place is a second home. I have seen the above backfire - especially when the ex is a 'Disneyland dad'. I have seen children get anxious and tense as they feel they raise their own behavioural expectations in response. I think what is happening is the child feels they must prove they are deserving of the attention they are being given. Is this at all familiar?

Does the teen have a guidance counsellor to talk to at school perhaps? Having an encouraging person he respects in addition to yourself might be beneficial. Would you be interested in talking to the guidance counsellor yourself to get some feedback?

Maybe too it might be helpful to enroll the boy in some extra-curricular activities. This could be a positive thing to share on dad's time. Seeing the child perform a range of activities might loosen the expectations of the father.