That CU would not meet modern regs, so by you now adding a new circuit he is maybe suggesting that it is a wise time to also upgrade to a new unit as it brings it up to modern requirements and allows him to sign it off as compliant.
I suspect he may be reluctant to add a new circuit otherwise.
I Have just had the same thing, but our unit is a bit older. My sparky (who I trust) also suggested a new compliant board when we have the kitchen rewired.