Your 9 year old son could love pooping his diaper which feels good.
That could be one reason your 9 year old has had a mess in his diaper the past few mornings.
It actually feels great to poop in a diaper and if he's wearing a diaper he might as well poop in the diaper during the night or in the morning.
There's no need to remove the diaper to go poop as the diaper is made for both pooping and peeing.
I would keep the 9 year old boy in diapers all the time and make him use the diaper for both pee and poop and lock the bathroom so he cannot get to the toilet.
He seems to enjoy pooping his diaper so if he's wearing diapers at night for bed wetting and pooping the diaper in the morning he should be made to wear diapers full time.
I would recommend Gentle Steps Size 7 diapers for your 9 year old to wear full time as they are good absorbent and comfortable diapers.
Also kids should always be in diapers on road trips too.
Yes older kids and even adults should wear diapers on long road trips.
Diapers on road trips are awesome because you can just pee or poop the diaper and then change later.
No need to stop for a toilet and no need to hold in your pee or poop when on the road trip.
I wear diapers on road trips and all the time as I no longer use a toilet.
Diapers are my toilet.