Go with your gut. If you feel your little guy is really "different" (attention span wise) then others his age, then have him evaluated by a professional. They will NOT diagnose ADD/ADHD at such a young age, but you will have some idea if he LOOKS like he is showing characteristics. The treatment for it at this age is behavior modification and a professional can help you with this.
He may also have some food intolerances as another person suggested. Unless he has really bad stomach pains and/or diarrhea he doesn't have food ALLERGIES, but certain foods (especially dyes in foods) can cause a lack of attention. You should read "Why is my child hyperactive" by Dr Fiengold...available on Amazon.com. If you are local you can borrow my copy (Ypsilanti Township). You should also google:
"food colors hyperactivity" and be prepared to be amazed!
I HAVE been where you are right now, my son is almost 6 and DOES have ADHD but we don't medicate him, we eliminated a lot from his diet (red and yellow colors set him off!) and added omega 3,6,9 supplements and set clear limits for him, pick our battles, enrolled him in karate, and above all...realize that this is his personality and everyone is different. It has been a long 6 years, but once I found some support from others in the same boat and realized I wasn't crazy (for thinking he was different)everyone felt better :-)
Good luck and please feel free to contact me off list for some more advice or to borrow some books.
J.