Hi, I would like to help you with your question.
It sounds like your relative may have a personality disorder. He does display traits of a few disorders, but which one is not clear. He also shows traits of other mental health issues, so I can see why you are having some trouble pinpointing his specific diagnosis. Some people have only certain symptoms of a disorder but not enough to qualify for a diagnosis. It usually falls into the NOS category, Not Otherwise Specified. Which means they have some but not all traits.
I can give you some ideas of what I think based on your information and some links to those ideas so you can research it further. You will be able to tell better from knowing him and having more exposure to his personality.
Schizotypal personality disorder is a possibility. Your relative does have some traits including having trouble relating on a normal level with people and behavior that is odd and not easily explainable. Running hot and cold is also part of this disorder.
He also appears to be passive aggressive in his reactions. He seems to like you one moment then ignores you another. Passive aggressive behavior usually is manipulative and is the person's way of trying to get unmet needs addressed.
He could also be manipulating you. Manipulative behavior is hard to recognize because the person uses it to make you feel you are causing the tension you feel in the relationship instead of them. They tend to be angry and are not able to express it in a healthy way so they act out instead. Manipulative behavior can be connected to narcissistic personality disorder.
The best way to cope with someone who acts out like your relative does is to not react. It sounds kind of strange I know! But when you react, it gives the person feedback about the effects they are having on you. Someone with a personality issue is going to feed off your reaction to give them satisfaction. So when they do something that makes you feel upset, it is a good time to back off and take a break. Go to another room, talk to someone else, or just take a deep breath and change the topic. Also, learn to filter what they say to you or any odd reactions they have. Let your relative "own" his reactions. See what he does as about him and nothing to do with you. Don't try to be responsible for an embarrassing behavior or apologize in any way for him. Here is a web site that may help offer further help in dealing with your relative:
I hope this helps you,Kate