When I was a kid I had big troubles with concentration. It was really, really bad - I could not sit still for even a minute! Thankfully, I had a very smart guy for a coach and he realized that the best way for kids to learn is via playing. He started giving me these two simple exercises/games at the end of our every lesson and I believe that they have helped me tremendously. These exercises are designed for kids, but they can certainly help adults too, so don't hesitate to try them! You might think that I am pulling your leg, but I am dead serious - these exercises help!
So, without further ado:
Extremely simple. All you will need is a stopwatch. Tell your kid to close his eyes (no peeking!). Whenever he is ready, you say "start" and start your stop watch. The goal is simple - your kid has to count to a minute in his head (eyes closed, no peeking, no words, nothing). When he thinks the minute has passed, he says "stop" and you check the stopwatch to see how much of an actual time has passed. So, say, your kid has yelled "stop" (meaning in his head count, the minute has passed) while in reality only 35 seconds have passed. This is the game-part of the exercise - you have to let him know that you want him as close to a perfect count as possible. Kids are competetive, they wanna do well and impress you.
So at first, his count would go something like this..
"One.. Two... Three.. ..... Fifteen... Sixteen... man, it's nice outside. I wish I could go play.. Wait, how much did I have? Ok, twenty one.. twenty two.. twenty three.. I am so going to get ice-cream after this! Probably vanilla, or maybe chocolate? Hm.. thirty three.. thirty four.."
You get the jist. This is how the thinking goes for people who cannot concentrate.
However, if you let your kid know that saying stop when, say, only 30 actual seconds has passed is a bad thing and maybe even offer a small incentive for "good" scores, he will start really trying to concentrate and calculate as thoroughly as he can. Bingo! That is what we were trying to achieve :) As a bonus, his sense of time will improve greatly!
This one is a little bit tougher, but not by much. Again, you will need a stopwatch, a pen and a sheet of paper. Your kid has to pick a three digit number (if he is really little and might have problems with three digit numbers, you can start at two digit numbers). Say he picks 345. Write that at the top of the empty sheet of paper. When you guys both ready, you say "start" and start your stop watch, while your kid continously adds 3 to this number, and writes the new number down.
So it would go: 345, 348, 351, 354, 357, 360, etc. etc. etc. He does this for exactly one minute (or two, or one and a half, it is up to you). After the minute has passed, you say your key-word (create one yourselves) after which your kid has to take the last number he had and start subtracting 7 from it. Say the last number was 360, so it would go something like this:
360, 353, 346, 339, 332, etc. etc. etc. Again for one minute. After the minute is up, you take his sheet and count all the entries that he made (you can check if they are correct but it doesn't really matter). Whenever you do this exercise again, your kid will really really want to beat his last result and therefore he will concentrate much harder. Vualia! We are helping him improve his concentration while also having some fun (and improving at maths!)
Again, I am dead serious about these exercises. If your kid, or you yourself, have trouble concentrating, try doing these regularly. It only takes a couple minutes after all :)
If you have any questions or if my explanations are unclear, ask in the comments and I will try to explain it in another way.
Best of luck!