He'll probably be intrigued by the whoopies, and impressed that that skinny little string will actually support his weight...easily.
They're not difficult to use...I just added them to my setup (and my son's) in Jan, and we've used them on our last two outings. No issues at all for either of us adjusting or using them. The only thing we do different than most is to attach the slings to the MSH via a lightweight climbing 'biner, rather than a toggle or dutch clips. No risk at all of the sling coming up over the knot or off the strap at all that way.