You have to give the food at least an hour, hour and a half before you figure the bg level is accurate enough to base a dose on. And then you can't be sure.
Are you thinking about this to see if the number goes down after feeding? It would be interesting to see, but you still have shootable numbers. So I am thinking possibly some pancreatic action but still needs insulin.
You could try this at night sometime - after +6, maybe a snack, then a test 30 minutes later to see if he went up or down?