Son continued his excellent season for Leverkusen in the 62nd minute when he collected a pass around 30 metres from goal, made space for the shot and scored with a dipping effort into the far corner.
Gladbach stirred into life in the final 10 with Leverkusen goalkeeper Bernd Leno twice saving from Max Kruse and Juan Arango. Raffael shot over the top and Gladbach had a penalty appeal waved away in the dying seconds.
Early in the game Leverkusen substitute Philipp Wollscheid was booked for throwing a spare ball onto the pitch, halting a Gladbach attack after they had taken a quick throw.
The win kept Leverkusen (43 points) on the trail of leaders Bayern Munich although they are still 10 points behind the defending champions, who visit Nuremberg on Saturday (1430 GMT).
Gladbach's season is in danger of unravelling. They have lost their last three league games and are without a win in their last five.
The Foals are fifth with 33 points, one behind Schalke 04 who play at home to Hanover 96 on Sunday, and in danger of falling behind in the race for Champions League spots.
(Reporting by Brian Homewood, editing by Tom Hayward)
