How to stop weird labmate from staring at me doing experiments without damaging work relationship (too much)? A few months ago my lab had a new member. Shortly after he joined I started hearing from students in other labs that he was being very weird, hinting that they were somewhat bothered. For instance he would roam around near other people's benches, stand really close to a student doing their work, or even follow them when they move to another machine. 

We are in an open-lab environment so a lot of labs share the same area. Maybe it was confusing to him and he thought it was fine to do what he did. Still,  I talked to my PI and he had the situation dealt with. However, when I am working he still likes to stand on the side staring at me. Apparently my PI forgot to tell him don't do that to your colleagues either. I know he's just being curious and wants to learn. I could probably use this as a precious opportunity to train myself to focus and ignore distractions. And I have learned over the years to tolerate some social behaviors in scientists which are not accepted by the general public. But his behaviors can be REALLY disturbing sometimes.

How should I talk to him about this without making the whole thing too awkward? I am afraid if I tolerate him any longer one day I would just blow up and yell at him... I don't want things to turn out ugly.
It's more of a comment, but I just want to voice that in my teaching career, I did encounter a couple people with these traits and they do have what we called Autism spectrum disorder (ASD). I am not diagnosing OP's colleague, but I'd like to suggest this possibility, and we should be mindful that it may not be fair to assume the person of question can fully function socially.

After talking to faculty and doing some homework searching for resources, what I found useful was i) talk to the person privately about your expectation, and ii) be direct, don't use social hints. In many ways it's not that different from what other users suggested you to do, but I'd just like to add another point that sometimes some people can appear to be rude because they don't know it's rude, not because they are rude.