How much calories does 300 lbs man burn with a push up?

The number of calories burned are closely related to what your heart rate is, and one single push up is too little, too short to create any important increase in your heart rate.

If you had the arm strength and endurance to keep on doing push ups continuously for say, 30 minutes, then maybe you'd go through 250-300 calories in that time. But not many would be able to keep going like that.

So assume that you do 10 push ups/minute, and that 30 minutes kills off 300 calories. That's 10 cals/ minute, and 1 cal/push up.