I don't think it is rigged. As an amateur comper myself, I've seen him in lucky draw events in which he actually didn't win. I once asked him how many cars he had won over the years, and he had to think for a few seconds before giving me an answer. How is he winning, this is my hypothesis:
For lucky draw:
1) Say every RM50 spent = 1 entry
2) Spend RM50 x 1,000 (RM50,000) = 1,000 receipts = 1,000 entries
3) Usually contests that open for one month (30 days) have 3,000 - 5,000 entries
4) Having 1,000 out of 3,000 entries belonging to you means you have 1/3 chance to win the grand prize
5) Of course I can't rule out the usage of fake receipts (I've heard of it from contest organizers, that's why some organizers ask for plastic wraps)
For highest spend:
1) Just spend RM20,000 and above (normal consumers can only dream to beat you)
Case study: Magnum Ice Cream (Unilever) was giving a Renault sports car as grand prize a few years back, and there was a leaderboard. Every ice cream wrap gives you a certain number of points, calculating backward, the winner spent around RM20,000.
In order to do the above:
1) You need money - I don't think he is from some broke-ass family to be able to pull this off
2) You need time - To buy things and write your personal details into every contest form
3) You need to be able to resell - I suspect he has a supermarket to resell the purchased goods (unless he is donating to charity)
I certainly cannot afford to do this because of a low-paying job and family responsibilities, but if you guys have the balls to team up, pool your resources and dethrone him, that would be interesting!