What about aspergers? YOu might ask your pediatrician about it if you are concerned, what has the school said about it.
I am totally not qualified to say, but I have ADHD and I've always been different too. I am not very social. I have trouble in groups, to the point where I have dropped college classes because of the anxiety I experienced in group projects. I never believed I could have ADHD becuase I had so many misconceptions about it. Researching the symptoms will probably be a great help to you.
Maybe you should talk to your doctor or school about getting him checked out. I would definitely see a psychologist before getting him on any meds. There are good ways to manage ADHD without meds. You might check out the site www.additudemag.com . It's a magazine but they have great info and symptoms on the site. Also, ADHD can manifest itself in hyper attentions, which could explain why he loves puzzles, which seems like it would take a lot of attention.
Good luck, and it's great that you are noticing these things and trying to get help. I wish someone would have done that for me. I suffered from it all my life and was not diagnosed until 6 months ago and I'm 34! It caused major self esteem problems for me growing up because I knew I was different from the other kids but didn't know why. I also thought I was a bad student, which is not true. Just knowing why I have some of the problems I do has been a great help to me. I'm just starting to