I think you will need to get out on your own a little first. Learn the areas you want to fish, then you will have an easier time getting him on fish. Read some of the articles on the Surfcating-Rhode Island Site. There are some good starting points in Joe's Articles.
http://www.surfcasting-rhodeisland.com/ He also brakes down the lures to use in your area.
Also you may want to try a charter. That will pretty much assure your son a decent size fish. I am sure he will have a good time.
Best of luck. I have a 3 yr old and a 3 mo. old and can't wait to fish with them.