Hi Jennifer
I agree with the Loss Adjuster appointed by Nationwide. What precautions could your son have taken to prevent the accident from happening? If there were no reasonable precautions he could have taken, I cannot see that negligence can be proved.
You could look at this from a different angle. Does your son have the packaging for the coiled spring garden plant support device? If the spring was used in accordance with the users guide/precautions and there was no warning that care should be taken when handling these while gardening, could you take action against the manufacturer? You could take action because either the device had a design fault (unsafe) and/or that the packaging did not contain sufficient safety warnings about use. If the packaging suggested wearing safety glasses while using product or gardening, then this would make any case difficult.
Do you have legal expenses on your own Home Insurance Policy. Give them a phone call to see if this would cover expenses for suing the garden product company. Some legal expenses under Home Insurances can cover expenses for £50k or more for pursuing cases like yours.The cover will under the policy you had at the time of the accident and not your current policy, if you have changed policy since.
If you don't have legal expenses cover, I would suggest discussing with Citizens Advice solicitor. http://www.citizensadvice.org.uk/
Due to the time that has passed since the accident, this will obviously make this more difficult but I believe you should pursue without entering into expense you may not recover.
Huckster