Hello! I'll do my best to answer your question/s...
If he is 5 years old, could be his primary front teeth...? I say that, as the permanent
front two teeth generally come in around 6 or 7 years of age. So, the fluorosis, seen & diagnosed today - may well be just on his primary front teeth.
With regard to will the fluorosis be on the permanents? Generally (sorry to tell you this) the answer is yes. But definitely NOT always. And reducing the risk, can be done by not exposing him to more fluoride of course, but it may not be enough to stop the fluorosis which is already systemically present and bound to show up - in time at least when the teeth are erupted.
Again though, until the teeth do come in, it will be impossible to say with certainty how much an effect the fluorosis has had on the permanents, and as mentioned, with some cases (the minority however), there is no fluorosis with the future new erupted permanent teeth.
I'm sorry I wish I could tell you more what you "want" to hear. But, that is generally what happens.
I should point out, that contingent upon what degree there may be fluorosis, many times tooth colored composites
can be done to mask the color of the flourosis. Occasionally even crowns
are employed, if the flourosis is severe. So, time alone will tell what, if anything needs to be done.
I indeed hope everything turns out the best for you & your Son Ma'am!
Please let me know if I can assist with any other concerns - OK?