You could:
A) Get a heavier, stainless steel can with the foot pedal to open it.
B) Wedge your can in between two spaces that he can't knock it over from.
C) Put the garbage can in a pantry, or closet. Or even get a garbage can that goes under the cupboard.
Or if none of those work for you, you could invest in the Ssscat. Its a can with a sensor on top that sprays air when motion is detected near it. You could place it near your garbage can.
Although, you could just get a sturdier garbage can for the same price.