The 55-year-old filmmaker has been talking about the project since 2009.
"The project on AIDS is on, but the script is not yet ready... [I had] too many schedules and things to do, so couldn't finish the script. I had discussed it with Aamir earlier and that time he wanted to do the film," the director told PTI.
The National Award-winning director will begin working on the AIDS film after the completion of his current project Kamaal Dhamaal Malamaal.
"I will complete the script first and approach him [Aamir] again. If he has time he will do it," the director said.
The untitled film will not incorporate the usual conventions of Bollywood, having no songs, comedy or traditionally commercial elements.
Filmmakers Revathy and Onir had explored this subject in Phir Milenge and My Brother Nikhil, achieving critical acclaim for their respective films.
Khan is currently preparing to make his TV debut with Satyamev Jayate, which has been described as his most ambitious project to date.