brrma95 I use an android app called ‘Data counter widget’. It creates a clean, simple data counter widget to your design.
You can try out the free version but in your case purchasing the Pro upgrade would be more useful because it allows you to set a daily quota limit and it only costs £1.69 (in June 2020).
Your son might appreciate the ability to customise the widget colours and text size.
I have it installed on my phone’s home page as I like to keep an eye on my data use.
No relation to the developer just a satisfied customer.