Haider was beaten by Pakistan Muslim League-Nawaz(PML-N) candidate Shaukat Khan.
Khan emerged victorious from a Multan seat by securing 11,868 votes, Geo News reported.
According to unofficial results, Shaukat secured 11,868 votes, while his rival candidate Haider secured 5,386 votes and stood second.
Ali Haider, 27, was kidnapped on May 9 while campaigning for polls in his hometown of Multan, often considered a stronghold of the Gilani family.
The gunmen shot and killed Ali Haider's personal secretary and a bodyguard when they resisted.
Gilani's other son Ali Musa Gilani also lost the polls.