I guess not so odd if he didn’t want his mother to know - especially since she works night shifts. This could have been amplified by the fact Richard was meant to be shielding - his mum may have been very concerned about him going out to meet people. JMO obviously.
It seems from what RO's Mum said she feels they have a close relationship.
I also think that many kids that age are very conscious of their parents expectations of them, some children desperately try to meet those expectations even though they are struggling and there maybe a conflict of what they want to achieve and what is expected of them.
Although this may not be the case with RO, this can put a huge amount of stress on youngsters, especially if they are seen as intelligent by their parents and don't want to let their parents down.
Covid has been especially difficult for young people and RO's Mum said he was struggling with his studies, had been neglecting his personal hygiene, it's understandable to become 'institutionalised' in one's room and become quite vulnerable.
I wonder about other friends too, he doesn't seem to have social media, so possibly a modest lad. unassuming young man?
Maybe RO wanted a change of scene, spend some time with some internet 'buddies', TikTok mates, a little freedom and space like we all do and did at that age.
Clinically extremely vulnerable pupils and students were due to return to their school or other educational setting from 1 April 2021.
Maybe RO doesn't want to go back, has gone to meet someone he knows or 'thinks' he knows (which is the worrying aspect).
I hope he's found safe and well.
JMO