You have a legal right to request a free evaluation by your local school district if you're concerned about your son's progress. Contact the Coordinated Campaign for Learning Disabilities at their toll-free number 1-888-GR8-MIND for information about warning signs for learning disabilities. Another excellent resource for parents is the Schwab Foundation for Learning. There are many support groups for families with children with learning disabilities throughout this country. There's a good chance that there is one in or near your community. The Coordinated Campaign for LD or the Schwab Foundation should be able to guide you to an appropriate local group that can give you the help you need.
In the meantime, have a look at Susan Hall and Louisa Moats' book, Straight Talk About Reading: How Parents Can Make a Difference During the Early Years. It's a great resource for ideas on how parents can help "jump-start" kids who are having difficulty with reading.