Something similar happened with my son. We homeschool and in 1st grade I had him read aloud to me every day. By the end of 1st grade he was reading solidly on a 5th grade level so I decided that the daily read aloud time was overkill. I didn't have him read aloud for most of 2nd grade and by the end when I did have him read aloud, he would guess at words and over anticipate. His comprehension also seemed to level off. I think he got used to skipping over hard words and guessing. I remember doing this myself at his age.
He is now just starting 3rd grade, and because I am compulsive, I am using REWARDS Secondary with him to reacquaint him with the process of decoding long words. I already see an improvement.
So this is a long way of saying that I think it's just a stage. If you want him to retain his oral reading ability you could have him practice reading aloud. It is a good skill to have.
