I just bought a 49cm road bike for my 4'10" son and just had to put a 60mm stem to make it fit. My mother is 5'0" and rides a 50cm. I could definitely see your son fitting a 52cm, MAYBE needing a slightly shorter stem. It sounds like he's going to grow a few more inches anyway.
That being said, if a 52 is that hard to come by (and/or you don't want to buy a bike without testing it), a 49/50 can probably be made to fit your son just fine. If leg extension is really an issue, you can probably get a longer and more setback seatpost.
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