I wouldn't strap him to the Spyder AT ALL!! :yikes:
Yeah, I know the chances of you hitting an immovable object, being hit by another vehicle, &/or getting flipped by/for whatever reason might be pretty small, but if he's strapped onto the Spyder if (or maybe when?) something like that happens, you be left responsible for his almost certain death - there's some very good reasons that 'bikes & quads & pretty much anything without some roll-over protection aren't fitted with seatbelts; just as there's good reasons that almost everything with some degree of roll-over protection does!!! You really don't want to have your Grandson strapped onto something that can end up on top of him &/or sliding along on its side with him held firmly in place between it & the road surface or another vehicle, do you?!? :shocked:
The Owner's Guide for these things warn that you shouldn't carry passengers who are too small/short to properly reach the foot pegs/plates and hold onto the hand grips properly; so ideally, you should probably wait until he is big enough to do both before giving him that first ryde! There are some threads here on the Forum showing how some owners raised the pillion foot plates so their short pillions could reach better, and (at least for a while?) there was also a set of brackets available commercially over your way that did that too - either could help?! :lecturef_smilie:
But if you must take him for a ryde now & he can't reach the pegs/plates & hold on properly yet, then I reckon you'd be much better off to strap him to YOU rather than to the Spyder - and preferably, strap him to your front, (like a tandem parachute harness) so that if the brown stuff does hit the fan, you can at least fold/wrap yourself around him & take the brunt of any impact..... it might hurt you a bit more, but at least it'd provide him some degree of protection - or maybe you could fit a roll cage with integrated seat belts? :dontknow:
Before their first Spyder Ryde with me, my Grandkids sat on my Spyder just about every week until they could show me that they were big enough to put their feet on the pegs/plates and hold onto the grips, and they've both been happily ryding with me at every opportunity ever since. Still, they weren't too much older than your Grandson is now when they finally got that first ride, and I reckon they enjoyed it all the more because they had to wait until they were big enough! It was a much anticipated treat that added to the eventual joy! :thumbup: