L.D.
First of all, check with your local school district and get him into their speech development program. It is free and they have been working wonders with my now 8 year old 2nd grader. Speech pathologists cost a fortune, so take advantage of your tax dollars. It is not an overnight fix. Be patient, do the exercises they will give you and give your son all the encouragement and praise. It is time consuming, but well worth it. L.