A good way to improve silent reading is to have your son follow along with his finger, while someone reads to him. He can point while reading silently by himself too, as this would help his eyes focus on each word. The reason that teachers always tell kids not to point (which, it turns out is not always good advice) is that it...slows...you...down! In your son's case, it's just what the doctor ordered. His teachers have said he just needs to slow down. Well, for some kids it isn't that easy! They need help with this, and they need to practice.
There are also computer-assisted reading programs that highlight each word or a group of words or a sentence at a time. Even very good young readers say that these kinds of programs help them focus and read with greater understanding. The difficulty is getting textbooks turned into print. This is starting to get easier, since a growing number of textbook publishers sell a CD with their texts just for this purpose. The time consuming method is to scan each page into a computer and then have computer "read" the material back (with the "highlight" feature on and the sound either on or off, depending on preference.) Ask the special education teacher or the reading specialist in your son's school (or a teacher of blind children) about software that reads text. They might already have it in the school. If this all sounds hi-tech, it is, but it's really not that hard to do and it's very effective.
Also, your son may be the kind of child who learns better auditorily (through his ears) than visually (through his eyes). We've all re-read a sentence out loud to gain more meaning from it. If this is your son's preferred learning mode, then leaving the voice turned on on a software-based text reader, or listening to books on tape, or continuing to read out loud (they'll need to find a place for him to do this, or have him read to younger kids) will be very helpful. Hope these suggestions help. Let me know.