I don't want to booster him at all... but I would like him to know the basics should there ever be an emergency.
Ie, if Mikah broke his leg RIGHT NOW and I rode in the ambulance with him, and DH had the car... GMIL would not be able to properly install a harnessed seat to come get him or pick him up from school. Lesser of two evils.
He's 4.5 and looks very rambunctious.
You say you won't have time when the baby comes.... however, there is absolutely no guarantee that if you 'train' him now, that he'll retain the memory any length of time for a hypothetical emergency situation.
I agree with the others, the nautilus is overall too large for him. There's a lot of wiggle space around that shoulderbelt, imo. Especially down his arm.
Personally,I wouldn't feel comfortable placing my child in it with that fit IF she wasn't a quiet settled child who I know would remain in place.
FOR an emergency situation, I"d look at a TB or Parkway (I think TB is more affordable) --- demonstrate its use to GMIL, then keep it at her location.
Then as you're driving, talk to your son and reinforce the idea over time that being in the car with grandma is a time to be as still as possible, like a mouse. Pretend he's with Grandma(you be grandma
) --- act as if he was with grandma in the booster seat for pretend. Do this maybe a few times a month. But no need to actually change seats.
Let him mature instead of worrying about training. This would be an emergency situation so hypothetical is okay. I honestly wouldn't worry that much about it at this point in time.
jmho