Why is galileo considered the father of modern science and physics?

Galileo is considered the father of modern science and physics because Stephen Hawking and other scientists believe that Galileo contributed more to modern science than any other figure in history. He is also considered to be the father of modern physics, astronomy and observational science. 

With Aristotle science was derived from everyday observations. This was how he came up with his notions of buoyancy based on shape and falling speed based on weight. 
Galileo disproved these with simple tests. He changed the focus of science from everyday observations to preforming experiments, something that modern science could not exist without. 

Examples of Galileo's accomplishments are seen in the legend where he climbed to the top of the leaning tower of Piza and dropped a canon ball and a wooden ball at the same time. These two objects supposedly dropped at the same speed, disproving Aristotle's previous notion that falling speed was based on the weight of the falling objects.

Galileo put a lot into the scientific method as well. 

Sources:Galileo government website, and high school. 

Hope this helps. :)