The insurers should be tackling this with the council, as Naomi's said. However, i seem to remember a glut of claims re trampolines in high winds a few years back - a load of cars were damaged over a particular period by 'flying' trampolines that weren't properly anchored. The result was that the car insurance companies paid out, rather than the house insurance companies, because it was an unforeseen event (act of god, they used to call it). I can see your son will be facing the same difficulty, but it really should be his insurers that are fighting for him- that's what he pays fees for...
Hope he's ok btw, that must've been a shock