Hi, I realize I am jumping in this discussion very late but wanted to respond anyway and find out how things were going. I was searching around the internet last night after my 3 year old daughter's pre-school teacher seemed concerned about behavior our daughter has basically done since birth. What you described in your son is very similar to her. When she builds, colors, does something on her own that she is particularly proud of she twists here hands back and forth or clenches her fists and presses them to her cheeks, while holding her mouth open. My mother has always seemed concerned about it, but I figured she would just grow out of it eventually. Now that she is coming into her school years I realize that it is not going away and that she may get stigmatized by teachers and other kids and I need to help her. I wanted to see if you had come up with the same diagnosis I had which is Sensory Process Disorder or SPD? When I first started my research I was coming up with a lot of people concluding Autism or "stimming" behavior and the relation to autism, but she has no developmental or social issues that would be consistent with autism. She is amazingly well spoken and this happened early, and she has met all her physical milestones on time or ahead of time. I was releived to see the treatment for this is not medication but play based occupational therapy. I found very useful news stories about SPD on youtube along with posted familar behavior videos of the behavior. There is a SPD foundation out of Colorado with very helpful information and resources. I am hopeful that your research lead you to the same conclusion of SPD and your son is doing very well, but if not this may be helpful information to act on now for him.
i think you are describing "stimming." it can occur in typically developing children, but can also be a red flag for other issues. However, since you do not see any other problems, i would suspect that he's just one of those 'normal' kids who does it. i wouldn't panic. however, you may want to mention it to the pediatrician and see what she/he says.