In fairness, neither the lampost or the drugs were mentioned until a lot of those replies were made.
So, in summary, he probably wouldn't obliged to report damaging his car, repairable or not, if no other person or property was damaged, to his insurer.
If he buys a replacement car, he will be obliged to declare that he had the crash but didn't make a claim, so that his new policy is based on the correct risk.
As he has hit a lamppost he is obliged to report to the owner of said lampost so that they can claim, and possibly also to the police.
If he was, as alleged, under the influence of drugs, then the OP and/or her partner, and/or anyone else who knows this, should report to the police.
The OP is obviously upset, and sounds frustrated at her DP and his son, but has confused the discussion by saying that the car was "written off", when it had just been declared irreparable by a garage, "lying" when this had not taken place, and not mentioning the lampost, and more crucially, the drugs, until people disagreed with her.
Other factors in whether this was a serious, irresponsible crash include what damage the car actually sustained, the car's age, weather, lighting, and particularly whether drugs were involved or not.