M.R.
Can you dig a little deeper with him? At 11 he can reason. I don't allow my kids to give me "I don't know" answers about anything. They have to come up with something to help me help them.
If he has bad ADHD, does he take a snack to his room and just forget it? That would be my first guess. Because that's what we ADD people do. I start with a cup of tea and I can't find it anywhere, then I forget about it, then I find it in the microwave when I go to heat up the next cup of tea I made that cooled off because I got busy doing something else while it was cooling off and now I have two cups of tea, so I set one aside to finish the other....and I do this many times a week. I'm not hoarding tea....I just forget that I had one going.