L.,
I would definitely get his eyes checked. If for no other reason than just to rule out a vision problem. You mention that he has motor skill issues. That may be it. If his balance is a little off, then he'll bump into stuff and fall. Does he trip a lot when he's doing something physical that requires concentration, like trying to climb a jungle gym? Has he ever, either through therapy or at play time tried to walk a balance beam? If he can do those things when he is concentrating on them, but trips over his own feet, then it might just be general clumsiness. I do stuff like that. Nothing wrong with me, just a klutz. For whatever reason, I simply cannot remember to look where I'm going.
When it comes to balance, an ear exam would be a good idea. Inner ear fluid imbalance is often the culprit for balance issues. Also, kids tend to trip over their own feet a lot when their shoes are getting too small. At this age, he's probably outgrowing shoes every 3-4 months. If he just started doing this, you may want to just stop at a shoe store and get him sized. A growth spurt might be the problem.
So, in short, reasonably anything could be doing it. I would try these first, then see your ped if everything checks out okay, but he's still doing it.
Jess