If the guy asks you a question in his message, give him a detailed response to his question and finish your message with a question that you are now asking him.
If you are nervous or worried about how to formulate your response, just try to think of the guy as though he were a good friend that you know.
Think about how comfortable you tend to be whenever you are around this friend and you are having a conversation with them.
When a guy is still using dating sites after having found a woman in real life that he is interested in, he is most likely not that interested in the woman.
In other words, there may be facets of her life and personality that he likes, but he may still believe that there is something missing.
By still using dating sites, he is indicating that he is still searching.