Once we understand the context, we'll be able to work out an answer. Notice three things:
- In this letter, pronouns are not capitalized when referring to God. There are numerous clear examples where "he" or "his" or "him" must refer to God, but they are not capitalized. So these words in the sentence in question may apply to men or God, depending on context.
- The broader context of this paragraph is on God's love and truth. God is described as "Eternal Love and Absolute Truth" in the portion you quote, and later in the paragraph, Jesus is said to be "the Truth."
- John 8:32, cited here, reads: "you will know the truth, and the truth will set you free" (ESV). "You" refers to the hearers of Jesus.
With this background, we can take a stab at the sentence in question. Here it is again:
Each person finds his good by adherence to God's plan for him, in order to realize it fully: in this plan, he finds his truth, and through adherence to this truth he becomes free (cf. Jn 8:32).
To make sense of "he finds his truth," we must determine to whom the pronouns refer. Let's start by identifying the easy ones as either God or Santiago:
Santiago finds Santiago's good by adherence to God's plan for Santiago, in order to realize it fully: in this plan, he finds his truth, and through adherence to this truth Santiago becomes free.
Now, remembering that God has already been called "Absolute Truth," and that God knows everything, it becomes necessary to interpret "he finds his truth" as:
Santiago finds God's truth
That is, the "truth" here is simply God's truth. God has a plan for each person, and each person's plan from God involves finding God's truth, and God's truth frees each person who adhers to it.