As a British citizen, he will not be eligible for the Art 50 EUV card as he was not resident in Austria as of 31.12.2020 – the onward freedom of movement does not exist – the Withdrawal Agreement rights would not apply to him. We are currently updating our content – and will have a lot for “3CN Brits” but are still trying to get the content together.
migration.gv.at might be a good starting point in the mean time.