One way to define modernity is to say that it is the time in which there is a move away from traditional ways of thinking and organizing society. It is a time when thinkers are moving toward an emphasis on rationality. Locke fits perfectly into this category.
Locke was very concerned with doing away with old ways of organizing society that were not, in his mind, rational. He felt that things like monarchy were based not on rational thought but on tradition. Therefore, he wanted to do away with them and institute governments that were based on the consent of the governed and on respect for fundamental rights.
Locke is a modern thinker because he wanted to set society up in a way that would be consistent with logic and rationality, even if those ways would clash with traditional values.