You say your son needs to be seen. Perhaps he doesn't need to be seen and a telephone consultation would suffice?
I and others I know have had telephone consultations where, previously, we'd have expected to be seen because the telephone option was not a 'thing'. The outcomes are the same - prescription issued, appointment to see GP, advice provided or onward referral.
Very few people, it turns out, actually need to be seen - as evidenced by many previous posters referring to 'empty' waiting rooms when they've been invited to attend a f2f appointment.
A lot of people can't seem to accept GPs are smart enough to diagnose over the phone when they have a patient's record in front of them, are listening to what the patient is telling them in the here and now, and asking them the same questions that would be asked f2f.
I truly also don't understand from where people have conjured up the idea the GP telephone triage system is about preventing treatment? To me, at least, it's perfectly straightforward. It's about prioritising those who need f2f and providing advice/guidance/support via telephone consultation if that makes more sense.
In your particular case regarding your son, the GP would listen to what you say and react accordingly. If a f2f is required, I'm confident they could accommodate an after school appointment.