You have to understand that is your son doing this only with his cousin or other people or friends around him. If no why he is doing this only with his cousin. Is cousin teasing him or some one else is doing similar behaviour at home/School which he is generally mimicking. Or may be this a cry for attention.
Observe what gains is he having that are maintaining his behaviour eg you would let him watch tv because he is disturbing the family.
What could you do is focus more on the good behaviour. Reward him more (specifically with your time & attention ) when he is showing good behaviour with his cousin. Please ignore the bad behaviour as much as possible.
If this still continue
Next Steps
Please meet a psychiatrist
Health Tips
You have to persistent in your efforts and never lose patience