I don't think he suffers from mental illness. There are indicators that he doesn't react in the normal way to things, and especially to reading people as people. That is the way he is, maybe, but usually, at his age, for example, people with ASD characteristics (and I don't like the disorder part of that name) know that they have it, and that this needs to be taken into account in dealing with people. I'm not saying he does have that.
How far people are responsible for their behaviour if they do have some sort of physical, or chemical, problem in relationships is an interesting point for discussion, but someone in a professional job (he is some sort of surveyor in building) should be able to modify their responses.