I too am appalled that he went to the barbecue over attending a police interview, but I would reserve my judgement on the mother at this stage.
I mean this with the greatest of respect, but how many of you talking about imposing sanctions, and not allowing him out etc, have actually had experience of dealing with a stroppy, arrogant, thoroughly nasty 15 year old who is bigger than you? If you have much younger children then you will no doubt be imagining how you would march them to the police station, and imagining how they would shamefacedly stay in their rooms because you told them to - but the reality is often very different when the time comes.
For all we know his mother may have tried really hard to get him to attend, but if he just doesn't give a fuck and pushes her aside and refuses to comply then there is little she can do. There is no mention of the father, and if he is not on the scene then it is pretty likely that a boy like this (who sounds angry, warped, aggressive, bullying and woman-hating) will be quite skilled at intimidating and overruling his mother.
Perhaps she is just fobbing the police off with excuses while trying to talk the boy round. Maybe she is a useless, pathetic arse who doesn't give a damn, and it's all her fault, or maybe she is a good mum, desperately ashamed of what he has done, but is out of her depth, ground down, fearful and in need of support in dealing with him.
Just sayin'.
(as an aside, if I were his headmaster I would have banned him from attending the leavers' barbecue anyway.)