One great way to teach it is by using math worksheets. One could make worksheets that are fun for children to complete. Adding graphics such as fruit and candy and having the children add them up will open their minds to it and have them learning in a fun way. Also, the sense of accomplishment the child has after completing the math worksheet correctly will encourage him to continue learning the subject.
If you find that your child is still really struggling after a number of efforts at home, there is one final thing you can do. The option of hiring a personal tutor is becoming more popular throughout the States. Personal tutors can help your child get to grips with things they never used to understand and provide a more personal form of education. The personal tutor will become more of a friend to your child, than a teacher, meaning the teaching style will be vastly different to that of their school. It's one of the best ways of helping a struggling child.