When starting any project, including helping your child with math, it is important to know WHERE to begin. If you are going to set and achieve goals, then it is imperative that you pinpoint where to start.
To determine what area you should focus on, use: 1) your assessment, 2) your child's assessment, and 3) your child's teacher's assessment of his needs. Let's start with number one, you. You should have a general idea of what area your child is lacking in through your helping him daily and his frustrations with the work, reviewing his graded work, and questions you've asked him, either through informal testing or oral questions. Based on your observations in these areas, you can determine what you think you should pinpoint.
Your child should be actively involved in the process, too. He, after all, knows better than anyone what he doesn't know. He may not be entirely sure what skills will help him the most, but he knows in what areas he's lacking. Use this. Ask for his input. Ask him what he feels would be most helpful for him to learn or get better at first, then take that into consideration.
The saying two heads are better than one really is true, and in this case three heads are better than two. Your child's teacher plays an integral role in this puzzle. Share with him you and your child's concerns about his mathematical abilities. Then ask him what he feels would most benefit your child in order of importance.
Once you have these three viewpoints, you can determine one or two things to begin to work on. Be careful that you don't overwhelm you or your child by taking on more than three areas to improve at one time. You and/or your child may become so frustrated and give up. By keeping the number of skills to work on down to one or two, you have the time and energy to focus on a small number, and are more apt to achieve your goal of improvement in these areas. You will also be more likely to choose more skills, once these are learned, to improve upon.