One other thought - if mom keeps whipping the kid she will get visited by the local authorities. At his age - that's call child abuse.
Let me add a bit more. Stealing by an ADHD child is not unusual! The child will act with out thinking. Many times they maynot even be aware they have taken the object. Punishment will not work!
The most important thing you can do to help him is to understand what ADHD is and how it is treated. This article from the WebMD. is a must read. It will give you a feeling for what you are dealing with. It seems fairly unbaised. Copy and paste it into your browser.
http://www.rxlist.com/attention_deficit_hyperactivity_disorder_adhd/article.htm
The second most important thing you can do is get professional help. Your school should hopefully be a good resource.
Hi nice grandma
You know this kind of punishment doesnt work with anyone, certainly not with ADHD kids.
My son is almost 18 he had very bad ADHD, I found that small consequences work better, like get him to give something back or help out. By being so hard you will make him 'defiant' and also you are giving him a lot of attention for the negative behaviour. You should focus on rewarding him for the positive.
Most kids try stealing at some stage. A short punishment, like get him to return the stuff, a talk about it. Then try to move on and forget about it.
The only thing that works in the long run is kindness, love, acceptance and understanding, this is the truth. Try to find out is he OK.
Good luck with this!
What is it your grandson does that causes his Mom to ground and whip him, could you enlarge on his behavior for us to understand , you say he is sweet so perhaps his Mom is tough on him? what is it he steals and why, have you had a quiet chat wiith him to ask him,that could be a good idea , get his viewpoint and ask how he feels.