First of all, God Bless YOU for taking on this great cross and love that you are giving your grandchildren. I have raised a son with ADHD and it brings many many moments of frustration and sadness for all. HOWEVER, if you begin to understand where this young man is coming from (how he is thinking - or better - not thinking, it will help you understand why he does what he does. There is so much to say that I could not possibly say it in this response. My advice is to go to the library and begin to study and read about Attention Deficit Disorder with Hyperactivity so you will know what you are dealing with. These children need clear rules (what's okay and what is not okay) and they need as little stimulation as possible.
I believe diet also has a big impact on how they do each day and I suggest you eliminate sugar in all forms as much as possible - this would include white bread, white potatoes, chips, etc. and most processed food which always contains high levels of sugar. Fruit also has a lot of sugar as well as all fruit juice drinks so give sparingly.
Also, know that these children usually do better one on one and even with you, his grandpa and his sister he will feel compelled to be the one who should get the attention, always.
Have definite routines for how things are done. At ten,you should have definite daily routines, he comes home from school, he has a rest period (remember he has been sitting still all day at school and it drives him crazy to sit in one place and be still) so, give him a couple of hours to play and unwind from his stressful day. After dinner have a period of time you or grandpa, or his sister help him with his school work. You will probably have to sit with him doing something else yourself so you are both there together as he completes his assignments. Always praise him for what he is doing right no matter how small as he is getting told how bad he is constantly by his family, his friends (which he will eventually have few of) and his teachers. It does not take long before he is convinced he is the bad child everyone thinks he is.
These children needs tons of love and patience and they need to be told very often how much they are loved as they are getting so much negative feedback from everyone.
Try to get him into some type of sport which will help him with his excessive energy, such as soccor, basketball, baseball, at ten, it is not to late to start this. ADHD in my opinion is a mal-function of the brain where the neurons do not contact as they should - you and I have a thought and we think "Oh, that would not be a good thing to say", unfortunately the ADHD child thinks the thought and because of the mal-function - eithers says the thought (which may be socially unacceptable) or does the act which is also socially unacceptable) either way he loses. After a while it is very difficult to keep explaining away these happenings.
The good part...these people (I am married to an adult ADHD) grow up to handle many things at once, they are usually highly intelligent (have higher IQ's than average) and are capable of great things. They do not do well in school settings but once out and into the world where they can focus on things they desire they do quite well; it takes a while before this happens however, 25+ years.
There is a school in East Dallas called DALLAS ACADEMY on Buckner, which is a private school for children with these issues, they will help your grandson feel good about himself and oftentimes turn these kids around and into successful students. They have a completely different approach to schooling. I highly recommend you check them out and do whatever you have to do to afford to get your grandson into this school.
My son who has the ADHD is adopted so he did not inherit his attention problem from his father, it probably has helped me understand him since I have watched how it affected my husband. My son, now 25, is the most loving, kind, patient person you could imagine, his youth and his troubles have made him tolerant of all people and their issues, he accepts what comes his way with maturity and love. He knows how it feels to be unloved and teased so he gravitates to the less fortunate.
Good luck to you, get to the library, learn all you can about this, and God Bless.