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These verses below should shed some light on your understanding.
Is it needful for someone who is not capable of understanding what sin is, to be Saved from the wrath of God concerning his sin?
Luke 5:32
I came not to call the righteous, but sinners to repentance. Do you believe in your heart, your son is or will ever be a Sinner?
Rom 3:23-26
23) For all have sinned, and come short of the glory of God;
24) Being justified freely by his grace through the redemption that is in Christ Jesus:
25) Whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God;
26) To declare, I say, at this time his righteousness: that he might be just, and the justifier of him which believeth in Jesus.
Scripture says ALL have sinned, but it understood the ALL spoken of, have the knowledge of Good and Evil residing withing their Sinful Nature.
Jesus is the Propitiation for sins that are past, having never committed sin, your son has no need to be Redeemed from them, he is Justified by the GRACE of God.
You are 100% correct, and not having that ability, he is not held accountable, God places a gentleness in the hearts and minds of those those precious children who so much has been taken, they are examples unto "US" the so called normal, on how we are to treat our fellow man.
The Love of God resides within their souls.
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Here are some verses which I believe speak to your questions.
Rom 2:11-15
11) For there is no respect of persons with God.
12) For as many as have sinned without law shall also perish without law: and as many as have sinned in the law shall be judged by the law;
13) (For not the hearers of the law are just before God, but the doers of the law shall be justified.
14) For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves:
15) Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another
Your son does naturally, those things contained in the Law.
To put it as simply as possible, in words you can understand without having to interpret Scripture.
Adam and Eve were in the Garden, they were told to not eat of the Tree of the Knowledge of Good and Evil.
They ate the fruit of that Tree, they fell and mankind was cursed with that Sinful Nature acquired by Adam, having had it passed on to his descendants.
NOW, concerning your son, has your son partaken of that forbidden fruit of that Tree?
Does your son have within him, the Knowledge of Good and Evil, as you and I do?
Is your son innocent in his understanding, will he ever truly comprehend Good and Evil?
Don't let you heart be troubled, your son has his place in the throne with You, Jesus, and God.
John 14:20
At that day ye shall know that I am in my Father, and ye in me, and I in you.