How to dissolve repetitive, self-condemning thoughts

Hi, urgent help needed. My 16 yr old son has had these  types of thoughts for over a year now and although we try mindfulness, CBT type strategies, lots of listening and hugs, the thoughts are building- GCSEs are about to start. School is very supportive, thankfully and has a staff member that he goes to. Son is also resistant to doing all but the parent listening and hugs and can't apply any CBT logic. Has anyone got any top tips please? Maybe 'self compassion meditations' that are good? How do we break this?