That truly sucks.

I was speaking to my own experience. Across an iPhone, Watch, two laptops (one corporate managed), and an iPad, it all 'just works' so to speak.

I've seen your issue before and it usually is because your non-iMessage phone number is associated with the same contact record as your iMessage computer account (probably your email). The iPhone will preference sending an iMessage over a text, so if your computer is on, you won't get a text. Sadly there is no 'force-sms' setting so the (admittedly awful) workaround is to create a second contact in your son's phone that is just your phone number.

It is indeed bullshit that it doesn't work for you.