Baboon, “Even” is an adverb here. It is a way of providing emphasis when you are making a comparison. For example: “David knows even less about it than Jane (does)” This sentences suggests that neither person knows this subject at all but that Jane knows slightly more. Without the word “even” the meaning is less clear and we don’t know how large the difference in knowledge/understanding is. For example: “David knows less about it than Jane (does)” could mean that Jane is an expert in the subject and David knows nothing. The first sentence in your question is correct (suggests that he is normally quiet but that he is being more quiet than usual - there is a contrast between his usual behaviour and his behaviour now). The second one does not make sense because there is no comparison in this sentence (he is odd compared to himself?) Hopefully, this makes it a little clearer.

including