Now a mediator is not a mediator of one, but God is one. (Galatians 3:20)
For there is one God, and one mediator between God and men, the man Christ Yahushua, who gave himself a ransom for all, to be testified in due time. (1 Timothy 2:5-6)
1) Is Yahushua the only "son of God" in the Bible?
No, in the Bible we find many sons (and children) of God (Yah). For example:
And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them, that the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose. (Genesis 6:1-2)
There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown. (Genesis 6:4)
Now there was a day when the sons of God came to present themselves before Yahuwah, and Satan came also among them. (Job 1:6)
But as many as received him, to them gave he power to become the sons of God, even to them that believe on his name. (John 1:12)
For as many as are led by the Spirit of Yah, they are the sons of God. (Romans 8:14)
For the earnest expectation of the creature waiteth for the manifestation of the sons of God. (Romans 8:19)
That ye may be blameless and harmless, the sons of God, without rebuke, in the midst of a crooked and perverse nation, among whom ye shine as lights in the world. (Philippians 2:15)
Behold, what manner of love the Father hath bestowed upon us, that we should be called the sons of God: therefore the world knoweth us not, because it knew him not. (1 John 3:1)
Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is. (1 John 3:2)
2) Who are these sons of God (Yahuwah)?
a) Angels - Genesis 6:1-4; Job 1:6
b) Men - John 1:12; Romans 8; Philippians 2:15; 1 John 3; and more...
The word "son" was employed among the Semites to signify not only the affiliation, but other close connection or intimate relationship. Thus, "a son of strength" was a hero, a warrior, "son of wickedness" a wicked man, "sons of pride" wild beasts, "son of possession" a possessor, "son of pledging" a hostage, "son of lightning" a swift bird, "son of death" one doomed to death, "son of a bow" an arrow, "son of Belial" a wicked man, "sons of prophets" disciples of prophets etc.
The title "son of Yah" was applied in the Old Testament to persons having any special relationship with Yahuwah. Angels or just and pious men were called "the sons of Yah" (Job 1:6; 2:1; Psalm 89:7; etc.). The leaders of the people, kings, princes, judges, as holding authority from Yah, were called sons of Yah.
In similar way we read about the nation of Israel as the son of Yah:
And thou shalt say to him: Thus saith Yahuwah: Israel is my son, my firstborn. I have said to thee: Let my son go, that he may serve me (Exodus 4:22).
3) How to become the son or children of Yah?
But as many as received him, to them gave he power to become the sons of Yah, even to them that believe on his name: Which were born, not of blood, nor of the will of the flesh, nor of the will of man, but of Yah. (John 1:12-13)
For as many as are led by the Spirit of Yah, they are the sons of Yah. (Romans 8:14)
That the righteousness of the law might be fulfilled in us, who walk not after the flesh, but after the Spirit. (Romans 8:4)
For they that are after the flesh do mind the things of the flesh; but they that are after the Spirit the things of the Spirit. For to be carnally minded is death; but to be spiritually minded is life and peace. (Romans 8:5-6)
But ye are not in the flesh, but in the Spirit, if so be that the Spirit of Yah dwell in you. Now if any man have not the Spirit of Christ, he is none of his. (Romans 8:9)
And if Christ be in you, the body is dead because of sin; but the Spirit is life because of righteousness. (Romans 8:10)
But if the Spirit of him that raised up Jesus from the dead dwell in you, he that raised up Christ from the dead shall also quicken your mortal bodies by his Spirit that dwelleth in you.
For as many as are led by the Spirit of God, they are the sons of God. (Romans 8:14)
The Spirit itself beareth witness with our spirit, that we are the children of God: And if children, then heirs; heirs of God, and joint-heirs with Christ; if so be that we suffer with him, that we may be also glorified together. (Romans 8:16-17)
And when all things shall be subdued unto him, then shall the Son also himself be subject unto him that put all things under him, that Yahuwah may be all in all. (1 Corinthians 15:28)
Conclusion: Because Yahuwah is a Spirit, His Spirit will be all in all. All, who are born of His Spirit, led by the Spirit, walk after the Spirit - are His children.
God is a Spirit: and they that worship him must worship him in spirit and in truth. (John 4:24)