Yes you should make your 8 year old son wear just a diaper and shirt while sitting in a highchair and eating.
Your 8 year old son will be more comfortable in his highchair wearing just a diaper and shirt and you can check to see if he's wet or poopy much easier as well.
If he poops his diaper you can just let him finish eating and then change him afterwards.
Here's some pictures of an 8 year old wearing just pampers diapers, shirt and socks and sitting in a highchair.
