He may have visual integration issues - or just age appropriate visual integration. My middle child learned the alphabet at 2, but didn't learn to read until a bit before 5 and we had significant struggles with any book that was in a crappy font, poorly laid out, etc. Even at 8 she's still restricted by font size and text density on the page, what she's ready to read comprehension wise is well ahead of the amount/density of text her eyes can manage on a page. Tracking is just too hard for her otherwise. My eldest used a bookmark as MegMeg describes until about 10 I think, and did not become a prolific reader until that age. Youngest child had the alphabet and was sounding things out on signs in public at 18 months but still isn't reading at 4.5... I imagine she will take off shortly like her older sister did and I am pretty sure that physical eye readiness/eye teaming/tracking is a major impediment for them.
