^What he said!
Definately the least hassle, and if your lad can handle driving about in a dented car for a bit then he can get it fixed later.
If you go through insurance you'll probably have to pay excess anyway so you're forking out regardless.
Who's at fault obviously matters, but the problem is proving it. Personally if it were me I'd agree to pay cash for the damage (subject to presentation of a quote or being physically present at time of estimate). If he doesn't agree to that as a suitable settlement i'd just say youre not accepting blame and to go through your insurances and let them drag it out and invonvience him.
I would think very little inconvenience to the X3 driver, probably fully comprehensive, curtsey car etc.