The fact that it will enable the son to look in the garden is not a factor as we have no right in respects of not being overlooked or invasion of privacy.
The reason is that we are a relatively small island and such a right is not feasible.
They can, however, argue that it will present a nuisance not just to them but other neighbours.
I would not recommend that they go beyond a letter threatening action for nuisance, because I think a court would take the view that it is not much different to a teenager playing music, a basketball hoop , playing football all the time or numerous other noisy teenage activities.
As such, I am afraid there are no magic solutions beyond the hope that a stroppy letter from a solicitor in respect of "nuisance" will put them off.
If there are any further points please reply and I will be happy to response.