Why does an acquiantance act this way towards me? nervous and weird...?

I have a guy friend whom I have known for about 3 years. We are not that close but he often asks me to tutor him with academic stuff. All throughout the 3 years that I have known him for, he has always been a confident, outgoing, popular guy, he often smiles a lot. But recently I have noticed that he has become very nervous, and shy around me. For example, one time we were in his car, he was driving and he nearly crashed his car twice (once, he drove onto the kerb while reversing and another time he nearly crashed into the car in front of him). Normally he is a very good driver. When I looked at him, he was smiling slightly and said "oh sh*t, i nearly crashed into the car in front".

Another time we were having lunch together, he dropped his coke bottle lid on the floor while listening to me talk and was slouching, staring down at the floor, and was so shy and quiet. It was so weird cos normally he is outgoing and friendly. Another time I saw him at my workplace so I went up to him to say hi and I talked to him. Normally he says hi back but that time he suddenly turned his body away from me and leaned on a nearby table, then talked to me from sideways?!

Another time I was tutoring him, I told him off because he asked me for advice before but he didnt listen to my advice, so as a result, he didn't pass his test. So I asked him "why didn't you listen to my advice?". I expected him to laugh it off but instead he started fidgeting with the book in front of him, hard out, and he mumbled something like "we all make mistakes". That doesn't match his usual confident manner. Also we were in a tutoring session talking, when he started laughing out loud awkwardly throughout our conversation for no reason, he kept scratching his head and looking embarrased.

Does anyone know why he's behaving so strangely? I dunno if he is like that around other people too, as I haven't seen him with other people much. But I've definitely noticed a change in his behaviour. I got worried and sent him a text message saying "i noticed you were really nervous, why is that?" and he replied "because I was self conscious, I realised that there are a lot of things that I should know that I didn't know". I assumed he was referring to our tutoring session. But it doesn't make sense because he was nervous while in the car, while having lunch and at other times too! I mentioned that to him but he didn't reply back. He's nervous and shy on several occasions. Whenever he asks me a question, he slouches, looks down at the floor and becomes so shy and nervous! Normally he looks me in the eye and asks me!

Does anyone know why he's behaving like this? Any opinions would be appreciated thanks :)