I don't know about the UK, but FB US has a process to request photo removal of your child, 13 and under. This includes hospital patients or anyone with capabilities issues.
Facebook Help Center
There are online forms for reporting. I feel sure you can search from the help page.
If your child is 13 - 17, they have to request it, although you can assist them.
And of course, anyone can ask for their own photos to be removed.
You have every right to do this, even if he gave permission; he may not see the
harm. Child privacy laws mean the request to take photos down should take precedence over someone giving permission.
It is incredibly easy for someone to identify and locate someone from a FB photo. Things like school, club or team photos or t shirts are part of the problem.
They might consider posting photos taken from a distance, the back, or an angle that doesn't show facial features.
Sorry if this is alarms anyone, but taught computer education to elementary-aged students for 14 years. One (true) story I shared every year happened locally. A 13-year-old girl was targeted by a man claiming to be 14. She met him on a teen chat site, and he used photos she sent to identify her location. He arranged to meet her in secret at her house. I guess when she saw he'd catfish her, she changed her mind. Police had been monitoring his activities; he was struggling to pull her out of a second story window when they arrived
I retired in 2018, and I shudder at the thought of all that kids face today.