Real Madrid forward Bale returned to the North London club on Saturday having agreed a season-long loan deal, though he might have been having second thoughts when Danny Ings scored at St Mary's to give another of the Welshman's former clubs a 32rd-minute lead.
However, Son levelled before half-time following a cross from Kane, who then provided another two assists for the South Korean to complete a hat-trick just after the hour mark.
The duo then combined for a fourth time as Son scored again in the 73rd minute before Kane added a fifth prior to a late penalty consolation from Ings, with Spurs claiming a win that must have left Bale, currently out injured, itching to make his comeback.
4 - Harry Kane has assisted all four of Son Heung-Min's goals against Southampton today - the first time in @premierleague history one player has assisted another four times in a single match. Fabulous. pic.twitter.com/EloGFWjEyE— OptaJoe (@OptaJoe) September 20, 2020
The game began at a frenetic pace as three goals were ruled out in the opening half an hour.
Kane thought he had scored a third-minute opener only for VAR to spot an offside against Son in the build-up.
Shortly after Hugo Lloris brilliantly saved from Che Adams, the Frenchman failed to clear his lines with a bizarre header when rushing out and Ings applied the finish, only to be denied as the ball struck his arm first.
Moussa Djenepo struck a post and Kane had another effort chalked off for an offside against Lucas Moura before Southampton scored the game's first legitimate goal, Ings collecting Kyle Walker-Peters' ball over the top and rifling into the net.
However, Spurs would go into the break level when Son turned home Kane's cross after good work from Tanguy Ndombele - who was surprisingly substituted at half-time having made his first start since March.
Son's touch to steady himself and his finish were Tottenham's only touches in the Saints' box in the first half, and their next made it 2-1.
Again Kane found Son and this time he scored with his left foot to put Jose Mourinho's side ahead less than two minutes after the restart.
The pair were not done, though, and Kane found his fellow forward twice more, with Son beating Alex McCarthy on each occasion to take the game away from Saints.
Kane got the goal his performance deserved too, following up in the 82nd minute after substitute Erik Lamela's effort came off a post, meaning Ings' late penalty - awarded following a handball against Matt Doherty - only reduced the eventual margin of defeat for Southampton.
What does it mean? Did Spurs really need to strengthen in attack!?
With leading striker Kane involved in the quintet of goals, and Son scoring four, it does not appear as though Tottenham's issues are in attack, despite the much-hyped signing of Bale.
Kane was the first player to be directly involved in five Premier League goals since Liverpool forward Mohamed Salah in March 2018. Bale is only adding to an area of the pitch where Tottenham are strong, while defensively and in midfield they still appear behind their top-four rivals.
5 - Harry Kane is the first player to be directly involved in 5 goals in a @PremierLeague game (1 goal, 4 assists) since Mo Salah v Watford in March 2018 (4 goals, 1 assist). It's the second time Kane has done so (also May 2017 v Leicester). Quintet. pic.twitter.com/3c4UbYzeKi— OptaJoe (@OptaJoe) September 20, 2020
Kane and Son: The dynamic duo
Since Son joined Tottenham before the 2015-16 season, he and Kane have combined for 24 Premier League goals – more than any other pairing in the division.
They had six shots and six key passes between them and seemed to have a telepathic understanding to get behind Southampton's high line again and again.
Moura goes missing
While Kane and Son seemed to take Bale's arrival as inspiration to up their games, Moura, the other member of Spurs' attacking trio, was disappointing.
He had just 20 touches – the fewest of any of the outfield players who started – made only five successful passes and failed to have a shot or produce a cross before being replaced in the 61st minute.
Key Opta Facts
Since Ralph Hasenhuttl took charge of Southampton in December 2018, his side have squandered more points from winning positions than any other Premier League team (37).
All of Son's four goals for Spurs were assisted by Kane, the first time a player has assisted another player's hat-trick since May 2003, when Thierry Henry assisted Freddie Ljungberg's hat-trick for Arsenal against Sunderland.
Ings has netted the opening goal in 11 different Premier League games since the beginning of last season, more than any other player in the competition.
Tottenham have failed to keep a clean sheet in each of their last 10 Premier League matches against Southampton, their worst run against a single side since 13 games without a shutout between May 2001 and May 2018 versus Leicester City.
Kane is only the sixth player to assist four goals in a single Premier League game, and the first English player to do so. Indeed, Kane only assisted two league goals in the entirety of last season.
Ings has either scored (5) or assisted (1) in each of his five appearances against Spurs for Southampton in all competitions, registering more direct goal involvements versus Tottenham than any other side since his move to the Saints.
Mourinho is likely to make changes in the EFL Cup at Leyton Orient on Tuesday – which could mean a recall for the dropped Dele Alli – as they travel to North Macedonia to face Shkendija two days later. Southampton are next in action on Saturday against Burnley in the Premier League.