It doesn't say you are not required to be here for 5 years for C permit, it says they can't impose a length of residency in excess of 5 years for C permit (i.e. not the 10 years that applies to most non-EU). The language requirements still apply.

How long have you been on your B permits? If you're at 3.5+ years, it would be worth your son deferring for a year to get his C. B permits only allow 6 months' absence, and as he's presumably 18ish, if he loses residence, he may not be able to simply return to Switzerland - as an adult, he might well be considered in his own right and would then have to meet the new (old) rules for UK citizens.

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