This is fairly common with stimulant medications. For some people they help with hyperactivity and focus but for others they only help with the hyperactive/impulsive symptoms. Usually in those cases it's a good idea to get a second medication that will help with the focus. A lot of people find they need a combination of meds to get the desired results. I'd suggest talking to your doctor about getting something to help with focus.
Another option is to try a different medication that could possibly help with all his symptoms. I hesitate offering this advice only because it often takes people a long time to find a medication that works for them, if they find one at all. Since the Ritalin seems to be doing a good job with many or your son's problems you might want to stick with it and work on the focus seperately. These medications work so differently in everyone it is impossible to know what will work for who.