Overall, I think it is too early to be concerned; I think Dude's hypotheses have quite a bit of merit.
OTOH, there are things I would keep an eye on, such as the bilateral coordination--not because of any link with dyslexia (yes--since found to be bogus), but for purely OT/PT reasons. I would also be observant about what his actual level of reading decoding skill is, whenever he lets a hint slip out. With his oral vocabulary and cognitive language abilities, it is perfectly reasonable that his own decoding skills, even if they are, say, at a third-grade level, might feel too slow for the level of text that he wishes to access. At the same time, after two years of reading (though covert), some level of automaticity in word calling might be imagined to have emerged (i.e., faster, more fluent reading).
I wouldn't bother with formal testing of the cog/ach/neuropsych type at this age, though, as you will most likely be assessing compliance and time-to-fatigue more than anything else.
Waxing and waning: well, he's still in the age-range for which food jags occur among many NT kiddos. No reason that might not happen for other interests as well.
In any case, even if, worst case, there is something that needs remediation, it is highly unlikely that anything irreversible will happen in the next couple of years without him making you unmistakably aware of it by his distress first.
