K.W.
There are many, many things this could be. Your best plan is to start keeping a journal and become a student of your son. What does he do well? What does he not do well? Times of day? Write down everything and look for patterns. Generally speaking, with the kind of kid you describe, there will be many layers of issues to sift through, including...
sensory and sensory processing
nutrition and allergies
sleep
learning styles/disabilities
environment
other
And remember, all of these interact with each other as well! In each area, watch carefully and see what you observe. Do you think there might be a problem in this area? If so, go get him assessed in that area. If not, move on to the next area. You are actually the best person to identify what is going on with your child, because you spend the most time with him. There is no single professional who can do a comprehensive evaluation of him, though lots who can do a great job within their area of expertise.
I affirm your decision to keep him in preschool another year. If at all possible, you may want to seriously consider homeschooling him after that, at least until you get a clear definition of what's up with him. Schools often do well with kids with a specific diagnosis and identified challenges. They rarely do well with the nebulous, "something's weird but we don't know what" kind of kids.
Good luck.