To the OP: get the child riding belt online for when he gets a little older.
My son, when younger, always used to fall asleep when riding with me on the road. That's why I used the harness. Sometimes he would wear the harness under his riding jacket, so you couldn't exactly tell if he was strapped on. I used to get a kick out of the worried drivers that would point, honk, and gesture that he was sleeping while I would pretend I had no clue what they were communicating, so I would just excitedly wave back.
Quite seriously, though, you just got to use that extra dose of caution, realizing the consequences if you (or someone else) isn't paying attention.
Have fun riding with your son!