I hope your son and the others are unhurt.
It sounds as though there are two separate issues here. Who's fault was the collision; and was the other driver insured.
If the other driver wasn't insured, there is a fund that pays out in those circumstances. Unless he was also driving recklessly (and you have witnesses to that effect) I suspect that the lack of insurance and his reluctance to give his details (which is illegal) won't be relevant as far as blame are concerned.
As for the fault - it does sound as though both parties may have had a part to play, but difficult to judge without further information.
As for the insurance company, unless it is clear that one party is not at fault (eg when you are stationary and hit from behind, the usual assumption is that the other driver is at fault) they normally do split the blame ('knock for knock')
I hope it works out ok for your son.
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