On a quick note, it is impossible to target fat loss on your belly. You will lose fat everywhere on your body when you do lose fat, and as a male you will likely see fat on your belly stay longer than other places.
What is the best time to work out during a fast?
Considering Ramadan starts tonight, I'm going to assume this is the fast you are talking about....
If you're in a caloric deficit you will lose weight.
Any type of exercise that gets your heart rate up for a length of time will help you burn more calories and bring you closer to that deficit.
Boxing can be a fantastic form of cardio.
It's all what you put into it.
On that note, if you are inexperienced I highly recommend following a clear ...