If it is your neighbour then you are unlikely to be contacted by Test and Trace, the part that you do have to abide by law.
However, let’s just assume your neighbour is definitely the reason why you got the alert.
Then presumably the only reason why your son hasn’t been alerted is because he doesn’t have a phone which has the NHS Covid app with the track and trace feature turned on.
So, the system is flawed, as one of you is isolating when they (arguably) don’t have to. Or one of you isn’t isolating when (arguably) do have to.
This is the main reason why I have the app but have the contact tracing feature turned off.
The app (like many Covid measures) will only properly work it EVERYONE does it. All these half measures and do it if you want to measures are the reason why the Delta Variant has become dominant.
Covid measures only really work if they are compulsory.
If we have learnt one thing during the pandemic, it’s that you really cannot rely on the ‘goodwill’ of the British public.
I am including myself in that as well before anyone jumps on me. I am also only doing the things I ‘have’ to do. I fully admit I am part of the problem.