It's unlikely in the extreme that Caesarion knew any Egyptian at all, and more or less impossible that he was, by the time of his death, particularly conversant in it--certainly his first language, and the only language which he would have spoken regularly, was Greek. The only Ptolemid that we know knew Egyptian was Cleopatra VII, whom we know from Plutarch was the first Ptolemid to learn the language. Caesarion was in his late teens when he was killed, but he lived the first years of his life in Rome. Following that we know little about him--it's unlikely that he left Alexandria to accompany his mother to, say, Tarsus (where she met with Antony after Philippi--they had met several times already, of course). There's no reason to suppose that Caesarion was actually taught any Egyptian--Cleopatra seems to be a conspicuous outlier in that regard. Even if he did, however, it's unlikely in the extreme that he could converse particularly well in Egyptian, and pretty much impossible that he did so regularly. Egyptian was simply not the language of Alexandria, or any of the parts of Egypt where the Ptolemies frequented