Hi Romem, sorry to hear about your son I really hope he's okay. If you don't have an accident report from the school already getting one would be your first step. Ask them for a copy of the report and any images of the fence, if they haven't made a report you will need to ask them to do an investigation and provide one when the investigation is over.
Once you have the report and any images you can then look for an appropriate solicitor if you are thinking of making a claim on your sons behalf. If you're worried about the legal costs involved you can always look for a no win no fee service, they don't tend to have an upfront cost and would talk you through their fee structure. If there is any fees they are taken as a certain percentage out of the final compensation amount that would be awarded to your son/yourself on his behalf.