While this behaviour is typical of a child on the ASD, on its own it really doesn't mean anything beyond a little quirk. I had some worries of my own about DS at one point, and I did a HEAP of research on ASD's, and became very aware of what the signs are- its probably a good idea to have a little search around on the internet and see if you think your DS has any other signs which might lead to concern. A word of caution though- once you start looking, you find ASD signs in EVERYTHING, and its very easy to get paranoid where there i no cause to be. My suggestion is to write down your concerns as they arise and then take them to your doctor, who can refer you to a paediatrician for an initial assessment.
Don't worry, it really probably is just a quirk- yay for a tidy boy!!!!