What was the personal philosophy of Alexander the Great in his youth?

— My memory is all Greek to me too. —

But it seems that here we see mainly a slight slip-up in letters with a bit of retroactive reasoning, or perhaps a certain conflation of concepts?

The concept of photos seems unfamiliar.

The concept of pothos is not.

Especially in connection with Alexander:

Pothos

Pothos is the Greek word for "longing", a divine power (daimon).

In Greek myth, Pothos ("longing") and his brothers Eros ("love") and Himeros ("desire") were the sons of Zephyr, the westerly wind. Alternatively, Himeros and Pothos were the sons of Eros. Whatever their precise family connections, Himeros represented the desire towards something that was within human reach, and Pothos was the longing towards an unattainable goal. Since the object of this longing could only be reached in a better, more perfect world, it comes as no surprise that Pothos was associated with death. For example, the word is also used to describe the Delphinium flowers that were placed on tombs.note

According to the Greek author Pausanias (second century CE), the sculptor Skopas made statues of Eros, Himeros, and Pothos. They were exhibited in the sanctuary of Aphrodite in Megara.

Aristobulus, one of the biographers of Alexander the Great, seems to have introduced the Pothos-motif in the histories of the Macedonian conqueror of the Achaemenid empire. He and all ancient historians after him believed that Alexander's inner drive was a kind of longing to see foreign countries. One of the attractions of the word was that an author who used it, could leave Alexander's reckless behavior during battles and sieges and his outrageous drinking habits unexplained. Like his legendary ancestor Achilles, the famous hero from Homer's Iliad, Alexander the Great had chosen to be famous and die young.

It is possible that the official portraits of Alexander were influenced by the Pothos of Skopas. If so, the idea to link the king with a longing for knowledge was contemporary with his conquests.

For a deep-dive into Alex and pothos, also check out: "Pothos.org – All about Alexander the Great"

For the generalized question in the title of this post: we do not know much reliable specifics of his youth in terms of intellectual achievements. But we do know the name of his teacher. Making it perhaps a fair guess that Alexander was some kind of Aristotelian (notably, Alex's tutor was not yet that famous or accomplished when called to Pella)?

The often legendary and obviously 'just invented' information we get about Alexander's youth are one problem to consider, but a much too strong influence of Aristotle on the young man and his views must not be assumed either.

Especially interesting for this is a contemporary critique of this choice of education:

Unfortunately, I am told that you study the wrong type of philosophy. This pseudo-philosophy concentrates on eristics. Now, eristics may not be entirely useless; it is even a good thing for men who will never be anything but private persons and will only meet others like themselves in order to refute each other. For you who are destined to be a monarch and ruler of peoples eristic is entirely unsuitable. Don't forget your future rank, don't forget that you should think of yourself as superior to your subjects. Are you to engage in eristic disputations with your inferiors? Yours is to command, not to persuade; theirs is to obey, not to debate with you. I am afraid, however, the reports are true, and it is indeed eristics of which you are fond.

Here is my own program of education. We should learn to speak - viz. the kind of speeches which can be used in practical everyday affairs and those which will enable us to deliberate about public affairs. If you will pursue this kind of philosophy, you will be able to form a sound opinion about the future, you will be able to give proper orders to your subjects, you will be able to judge correctly what is good and just and what is not so, and you will know how to reward and punish.

Compare this program of education with what the sophists from the Academy have to offer. They will teach you to quibble and split hairs concerning problems of no practical value whatsoever. They will never enable you to cope with the actualities of daily life and politics. They will teach you to disdain opinion (common sense) in spite of the fact that common sense assumptions are the only basis for ordinary human affairs and they are sufficient to judge the course of future events. Instead of common sense opinions, they will make you chase after a phantom which they call true and precise knowledge, as distinct from mere opinion. Even if they could reach their ideal of precise and exact knowledge – it would be a knowledge of things entirely useless. Do not be deceived by their extravagant notions of goodness and justice or their opposites. These are just ordinary human notions not so very difficult to understand, and you need them only to help you to meet out rewards and punishments.

Sober up, therefore, give up your present studies under Aristotle and others of his ilk, and study the way I told you to. Only in this way can you hope to become another Philip in due time.

[Isocrates writing a letter to warn the Macedon court of the perils of their choices.] Quoted from
— Philip Merlan: "Isocrates, Aristotle and Alexander the Great", Historia: Zeitschrift für Alte Geschichte, Vol 3, No 1, 1954, pp60–81.

Note that only the Latin transcription lends itself to this letter switch from photos to pothos so easily:

Compare φωτός vs πόθος.

Given how "photos" was described in the question, my guess is that it is mixed up a bit with the Aristotelian concept of 'truth'?

Truth, in metaphysics and the philosophy […]

The correspondence theory

The classic suggestion comes from Aristotle (384–322 BCE): “To say of what is that it is, or of what is not that it is not, is true.” In other words, the world provides “what is” or “what is not,” and the true saying or thought corresponds to the fact so provided. This idea appeals to common sense and is the germ of what is called the correspondence theory of truth. As it stands, however, it is little more than a platitude and far less than a theory. Indeed, it may amount to merely a wordy paraphrase, whereby, instead of saying “that’s true” of some assertion, one says “that corresponds with the facts.” Only if the notions of fact and correspondence can be further developed will it be possible to understand truth in these terms.