Number of points from your post:
1) No matter which supplier you use, it all comes down the same copper wire to the property so changing the provider will not change the underlying infrastructure with whatever limitations that has.
2) You say that you are seeing a higher speed when you test using TBB as compared with your son testing using speedtest.net. That implies that the line is capable of achieving the higher speeds some providers are offering. To cover their backs some providers only offer a service based on speed test results. If your son is consistently showing results slower than his neighbours it is possible that some providers are capping their offer at what the line is appearing to achieve.
3) This is all starting to point to something sub-optimal in the kit or configuration used by your son. How were the speedtests used by you and your son connected (WiFi or ethernet cable)? WiFi (particularly in multi-dwelling units like blocks of flats) is liable to congestion; this can be alleviated by changing the WiFi channel. There is plenty of advice on line about checking which channels are being used, which are the most congested and how to change if the kit is running on a congested channel. Bear in mind that the congestion (and possibly speed test results) will change over the day as people come and go, start streaming films in the evening etc. What speed does you son get using the same speedtest as you? If it is markedly different to yours then there may be a problem with his hardware. As a final thought, are there any telephone extensions in the flat? Some extensions which have not been correctly wired-in can cause interference - and has your son tried changing the filter(s) if there are any plugged into the telephone line?
Given the facts as laid out in your post I would suggest a bit of background research before jumping out of the frying pan into the fire.
Edited by GonePostal (Sat 12-Mar-22 10:20:04)