He was beaten by Colin Clark, a hardline Brexit-supporting Tory, in Gordon in 2017 - in no small part thanks to (presumably remain) supporting Lib Dems and Labour voters voting against him (as some gleefully admitted to doing - there's one former Labour MSP who openly confessed he was delighted the Tories swept the north). He's too marmite to get back into parliament, which he's openly admitted would be his aim... if, by some sad realignment of the planets, Nicola Sturgeon and co had a bizarre change of face and let him waltz back into the party, which won't happen anyway.
Personally I saw him as yesterday's man not long after the referendum defeat tbh.