I'm with Judy, no clue about diagnosing or even what to do about these symptoms. I'd say it's a bit odd, but he is still 19 months so maybe he is just exploring his world? Lol in a strange way? No, really I can't be of any help...my only suggestion would be casually mention behavior to the parents. Like..."hey I noticed Timmy likes to rub hid head on the floor, that can't feel good. Have you noticed this at home?"
This might open the parents eyes to see something, especially if this boy is the only child...they might not know it's unusual.
Good Luck!