Had a detailed reply typed then my laptop shut down and lost it all. In "brief" pleased to hear your son is basically not too badly hurt but see how his shoulder goes before its sorted. One thing to watch is the insurance companies ways of screwing you. Your son will probably ( almost certainly) have a no blame claim against him now, worth checking. That will mean his no claims discount will remain intact but the overall policy cost may (probably will) go up. So in effect he will pay more, try and get that increase for the coming and future years covered by uninsured losses or by whatever means. Insurance figures have gone down this year by something like 17% so you will expect a reduction in premium on renewal. By someone else's action he doesn't want to be out of pocket next year or coming years. I guess he is reasonably young in which case the increase in insurance costs could prove significant.
From my lost message, record and log every communication, you may or may not need it in the future, dates/times/people spoken to. Remember all calls to insurance companies are recorded and that can be handy should you wish to confirm what has been said. You may be very lucky and it all goes great but cynicism is a combination of age and experience, mine haven't been ideal with insurance companies. I do have to say my wifes last little escapade was relatively painless with Sheilas Wheels but I expect the increase in premiums will be coming as it was her fault. I digress.
Whether you get your car repaired or not will be down to the assessor and his costing, you may have opportunity to discuss it though. If its written off make sure you negotiate a replacement cost. You are not being paid the bottom book price for a car which may have been low mileage in prime condition. If you have recent photos that would help as a smashed up car can look a bit sad in the repairers yard.
If it goes to a repair select where you want it doing, insurance companies have bulk repairers which most do a fair job but they are in place by the insurance companies to save costs, make your own mind up on that. I wanted mine covering by the manufacturers warranty and that had to be done by a VAG bodyshop. If the garage will give you a written warranty for the full repairs as a manufacturers warranty that's one thing as long as they stay in business. My car had to go back after a year and it was done via VW and no issue. I suspect I may have had an issue with an independent bodyshop.
Last point is that you will be hounded by ambulance chasers telling you that there is a pot of money available to cover injuries, I guess whiplash etc. So far I have had in excess of 66 calls and it gets annoying and that's after 2 years now with no injuries.
Weigh up what you will and will not be compensated for especially future premiums and determine how you may need to claim for your sons shoulder injury should it prove necessary.
I am biased against insurance companies as you may have gathered but that is only because they appear to suggest one thing then possibly do another, be very precise about what you want out of this, which I guess is a fully repaired car back as it was and your lad back on the road at no expense to either of you.
Good luck, hope it goes well.
Bob