If you haven't been playing board and card games with your son, start now. Begin with games that are easy to play before moving on to those with more complicated rules. Not only do games require concentration, but they're also fun.
Read to your son. Start asking him questions before you read to focus his attention on a story. Then have him let you know when you read the answer to the question. What you are doing is helping your son become an active listener.
Does your son play any computer games? Have him avoid those that just require mindless responses. Also, cut his TV viewing time. While it may appear that he is concentrating on a program, television does not require children to focus for more than a few seconds on an image.
Other activities that build concentration include: doing puzzles, drawing pictures, making models, learning karate, and helping with family chores. Your child's teacher will have even more suggestions.
Find out from your son's teacher whether his problems with timed assignments are due to not focusing on the work or not having the necessary skills. Since most timed assignments are math problems, your son may need to learn the basic facts better.