Dear P.B.,
It is difficult to explain what you might do without observing your son. Reading is a complex skill, and students can have lasting trouble for a range of reasons. It does sound as though there may be a particular obstacle in the way of your son's progress which will require clinical remediation. Some very likely suspects are inadequate auditory skills, naming problems, or other language deficits--since reading is understanding language which has been written down.
In addition, your son may have developed some anxieties or doubts about his ability to learn. A fearful or hopeless attitude can have as much impact as a deficit in learning skills.
You need a professional knowledgeable about reading who can find the level of your son's success, and rehearse with him there. Students need abundant successful rehearsal in order to develop fluent, automatic skills. Meanwhile the professional can get to know more precisely what is holding him back. Then the key is to isolate and build up any area of deficit. A likely example: If his sounding-out is weak or slow, find a speech/language pathologist who can assess and treat naming, phonological awareness, and the other basic auditory skills of language.
Best Wishes,
Ruth Alice Jurey, M.S.
Speech/Language Pathologist