Sounds like "Nature's End" by Whitley Strieber & James Kunetka.
- Explosive population growth
Ironically (as I type, the actual world population is quoted as 7.12 billion according to The US Census Bureau) this is set at 'over 7 billion' but the 'Depopulationist Manifesto' is quoted as saying:
The reason that life on earth is in danger of destruction is that there is an overpopulation of human beings.
- Widespread global pollution
The first chapter has the son (Tom Sinclair) of the protagonist John Sinclair dying trying to save others "in the Denver pollution catastrophe of 2021".
- One of the antagonists is a cult leader who preaches to people to voluntarily commit suicide to reduce burden on society
This character is Gupta Singh, the head of the Depopulationist Movement.
Every person who joins the Depopulation will gain much from his death and rise far in his next life.
- There is some backdrop story to this antagonist in which he is revealed to be a sadist torturing little animals
Once I got a cat and tied it by its tail under the window, and cut a small slit in its belly. It screamed and squirmed about, and once in a while I would stick my fingers inside -
- The protagonist races against time to find a solution for the pollution which is becoming lethal
This is the only thing that doesn't match. They are racing to complete a 'conviction' - a computer simulacrum of a personality which can be questioned and will always tell the truth about the person it is simulating.
- He uses notes from his late son who was a researcher in the field to follow a trail. The research notes are in a data pad that self erases if certain questions, devised by his son, are not answered when reading it
This follows the description of Tom's datafiles exactly, although I can't find the quote.
- The solution he finds at the end is a secret lab that has been growing super-intelligent next generation kids who have the potential to solve all the problems.
It's not a lab, but a secret community of hyper-intelligent children and uplifted apes called "Magic".
Incidentally, this is one of my favourite books! :)