if he can do you think 5ft can ever win

Sure, stand on a chair.

in the nuts

They most certainly can. go one foot higher on the scale & u can see Ruslan Chagaev who is 5'11 handed Nicolai Valuev who is 7' his first loss. In order to pull such a feat off the shorter man needs a distinct advantage in speed & skill as well as a strong game plan preparing when the smaller man will be able to move within the range of the opponent.
Another way of doing it is in lighter weights where the shorter man will inevitably be stronger & can drive the tall man back into the ropes & work the body until the head falls

The shorter boxer will not want to be a ''head hunter'' but will want to get inside and work the body. He will not want to stay on the outside but will want to fight inside.