I am a little bit confused by your question. For one thing Caesarion is not thought to have been poisoned and the Roman sources are explicit about his assassination (execution?) being carried out on the orders of Octavian. How accurate or comprehensive these accounts are is another matter but I think we can mainly focus on what they do say for the purposes of Caesarion's death.

Plutarch is the oft-cited source on this matter and while he is notoriously unreliable and did not intend his works to be used as historical annals in the way modern historians do, he really is the best thing we have had to work with.

According to Plutarch, Cleopatra feared for Caesarion's life if he was in Egypt when Octavian took the kingdom, and rightfully so given the extent to which she had built up his future legacy. Bestowing titles like "King of Kings" upon your child along with his monstrous inheritance as her successor and co-regent would paint enough of a target on their head but when that child is believed to be the biological child of your rivals adoptive father and predecessor...well, you may want to start taking the possibility of assassination into consideration.

Plutarch names a certain Rhodon as the tutor who betrayed Caesarion to his death but he does not imply that Rhodon actually went so far as to assassinate him personally.

Cleopatra's children, together with their attendants, were kept under guard and had generous treatment. But Caesarion, who was said to be Cleopatra's son by Julius Caesar, was sent by his mother, with much treasure, into India, by way of Ethiopia. There Rhodon, another tutor like Theodorus, persuaded him to go back, on the ground that Caesar invited him to take the kingdom. But while Caesar was deliberating on the matter, we are told that Areius said: "Not a good thing were a Caesar too many."

As for Caesarion, then, he was afterwards put to death by Caesar,—after the death of Cleopatra.

Caesarion's journey refers to a course taken overland to Ptolemaic controlled Lower Nubia (often referred to as Aethiopia in Greco-Roman sources) to one of the Red Sea ports which conducted trade with Arabia and the Indian subcontinent. That Cleopatra would set such an itinerary for Caesarion is more than plausible as it would take him far from the reach of Octavian, not to mention the Hellenistic and Roman influence which did exist in some parts of India. For him to settle in India with a sizeable fortune might have been quite comfortable but it was apparently not meant to be.

Rhodon advised Caesarion to return to Egypt and accept the crown that Octavian offered and Plutarch heavily implies that this transpired soon after Cleopatra's suicide. Arieus Didymus, a former patron of the Alexandrian court and new patron to Octavian is then either credited or blamed with convincing Octavian that the 17 year old was too great of a potential threat to live.

Cassius Dio is the other major source and he also states that Octavian ordered the death of Caesarion in order to prevent future strife. Dio mentions how Cleopatra sent Caesarion away for fear that Octavian would have him executed, however he does not mention the events surrounding Caesarion's death in as much detail as Plutarch. He also states that Caesarion was killed on the road to Aethiopia, a statement which is in all likelihood a shortened or corrupted version of the same story Plutarch recounts.

Of course, Plutarch's chronology has been questioned as Ge includes several known errors and there have been theories that he perished before his mother but without supporting evidence I see no reason to assume this.

Dio does not specify if Caesarion's advisors betrayed him or were themselves tricked into believing Octavian's offer was genuine. Plutarch meanwhile chooses to compare Rhodon to the tutor Theodorus who betrayed Marc Antony's son, Marcus Antyllus, to Octavian's men and then looted his corpse after his execution.

Interestingly, mentions of Octavian's intentions to parade Cleopatra through Rome in his triumph omit Caesarion entirely although it is implied that he always intended to include her children by Marc Antony, at least two of whom were forced to march through Rome.

This makes sense as it is extremely unlikely that Octavian would even want to march a young man who was thought to be his blood relative in triumph through Rome towards his eventual execution or imprisonment. While Octavian's claims that Caesarion was not the son of Caesar certainly had some effect, Dio believes them although Plutarch takes the opposite tone, it could still have been a potentially disastrous PR move.

After all, the Roman mob had a soft spot for Cleopatra's family and had been moved to mercy for both Arsinöe IV and the twins Alexander Helios and Cleopatra Selene.

Not to mention the potential damage that Caesarion could do if he somehow ended up trying to put his mother's kingdom back together. 17 certainly sounds young but conquerors had started at that age, even his own mother was probably not more than a year older when she began playing her own political game and trying to win the Egyptian throne. It is entirely possible that those who opposed Octavian, even those in Rome, or those who opposed Roman influence, might support him in the East where he had a powerful claim as the inheritor of the Ptolemies, Seleucids and Caesar. Maybe none of this would have happened. He might have survived to become a vagabond artist or a mercenary or a sickly man who died young. But that is a lot of doubt and a lot of risk for anyone planning on building an empire or a dynasty.

Why not just kill the boy and save everyone the trouble?

There is no evidence exactly how Caesarion died but violence is far more likely than poison. In fact, the death of Caesarion's close friend and stepbrother might reflect his own. As I mentioned Antyllus was betrayed by his tutor. He tried to find sanctuary in the Caesareum only to be seized and dragged before a statue where he was charged with treason and summarily executed on the spot by beheading. A bloody end justified by the fact that he had recently come of age along with Caesarion, and that the two had entered the military as a result (although this was mainly a PR move on the part of Antony and Cleopatra). If Antony's son and heir could be killed in such a fashion, so could the alleged bastard of Julius Caesar, particularly with such high stakes.

But again, we have no way of knowing exactly how Caesarion died, we can only say that Octavian likely ordered it as a preemptive measure.