The parking position of the third party is unlikely to make any significant impact on the third parties ability to bring a claim against your son.
The Road traffic act requires a duty of care on your son's part.
By hitting a parked vehicle, while carrying out his 3point turn. He has failed on this duty and therefore will be considered liable for the TP damage. That the TP, from your note, had parked illegally most likely, while a contributory factor, in that your son's 3 point turn might otherwise have been completed without incident, I don't believe will have adequate weight to do anything to shift the position of your son's Insurer having to accept liability in this instance.
Hitting something that is parked nearly always makes you liable, unless another party shunted you into the stationary object in the first place which was beyond your control.
Sorry not to offer more encouraging comments, I work in Insurance and this would be my work-minded view.
Negotiating cash isn't necessarily advice I'd advocate, but sometimes does lead to a more sensible solution. You need to consider the cost of your son's No Claims Discount (if he has any) the quality of car he hit, to get an idea on parts/labour costs and then decide if you wanted to make a "without prejudice" offer for the envisaged repair cost. Any offer you do make you should qualify as a WP offer, so in the event negotiations fail, you have tried to protect your motor Insurers position to some extent.
Some motor Insurers will get the hump if you attempt to negotiate a TP claim, fail, then want to pass things along as most likely your actions have prejudiced any ability for the Insurer to consider defending the claim.
Hope this is of help.
