This is a GREAT question!
I am 22 Years old and have had tourettes/ticks my entire life! I have researched everything there is to research as well as been on quiet a few different medications. I was acting as an advice coloumnist for "tourettes weekly" when I was a teen. It was a lot of fun because there are so many different types of ticks. Mine consist of scrunching up my nose, excessive blinking, and I have had a few more in the past that I have grown out of.
Assuming that physically easy child/difficult child has no physical problems with his leg/knee that would result in the bouncing motion, It could deffinately be concidered a tick. It could be a 24/7 tick, or a nervous tick, and there are many different types of medications that can help the situation. it may not get rid of it completely, but it will help him keep it under control, without even thinking about it!
I learned to love mine, even if they are different and was taken off of my medications. Although, my ticks were not disturbing to others (that I know of) I would suggest speaking with his PCP about different medications that can help him better cope and distract him less.
Please! let me know of any other questions, and good good luck to you and yours!