I would entirely agree with this EXCEPT that his family had been assured that his medical exemption would be valid. If they hadn't, they wouldn't have gone.
So they traveled under false pretenses and then were told "you can't go home if he doesn't submit to this". What followed most likely scarred the kid for life.
Ryanair shouldn't have told them there was a medical exemption until it was too late for them to change their minds about traveling.
I agree Ryan air should not have done that. However, that's actually not relevant to whether he could catch covid. The exemption certificate doesn't of course exempt him from catching it, and his family decided to take him somewhere he could catch it, or was at greater risk, especially on a plane with other people who may or may not have been exempted. This was also the cause of him having to have the first test. The family could have avoided all this.
I don't personally think there should be exemptions in any case, as this puts all on a plane at risk, doesn't it?