It's impossible to give a statement about which fence you own - you will have to look on the Title Plan of your property register and there MAY be an indication by the plan being marked by little Ts marked on the plan. Where the Ts are marked on your side of the plan, those are your fences.
This only applies to more modern estate developments - older properties won't have this.
Irrespective of who owns the existing fence, it is not an obligation to maintain it (unless a specific clause in the Title Register says there is), and it is perfectly permissible for you to erect your own fence on your side, or to merely device something just across the 'hole' to prevent your son from getting through.