The question essentially reduces to finding the antecedent of "him" (αὐτοῦ) the last word in John 1:7.
The answer to this question becomes clearer when the overall outline of the section is observed:
- John 1:1-5 - about Jesus
- John 1:6-8 - about John the Baptist
- John 1:9-13 - about Jesus
Now let us examine a literal translation (BLB) of John 1:6-8
- V6: There came a man having been sent from God. His name - John.
- V7: This one came as a witness, that he might testify concerning the Light, that all might believe through him.
- V8: He was not the Light, but [came] that he might witness concerning the Light.
Thus, the antecedents of all the pronouns here is John the Baptist as confirmed by V8; ie, it cannot be Jesus because Jesus, the Word, is the Light and V8 says that "he" was not the light. That "he" and "him" must be John the Baptist.
The "believing through him" is direct reference the explicitly stated purpose of the ministry of John the Baptist as "preparing the way for the Lord", Matt 3:3, Mark 1:3, Luke 3:4. That is, without John the Baptist's preparatory work, Jesus ministry would have been much less effective.
Compare this with Ellicott's remarks:
(John 1:7) For a Witness.—Stress is laid upon the work of John as
“witness.” This was generally the object of his coming. It was
specially to “bear witness of the Light.” The purpose of testimony is
conviction “that all men through him might believe,” i.e., through
John, through his witness.
Meyer is similar:
διʼ αὐτοῦ by means of John, so far as he by his witness-bearing was the medium of producing faith: “and thus John is a servant and
guide to the Light, which is Christ” (Luther); not by means of the
light (Grotius, Lampe, Semler), for here it is not faith in God (1
Peter 1:21) that is spoken of.
The Cambridge commentary is similar:
through him] i.e. through the Baptist, the Herald of the Truth. Comp.
John 5:33; Acts 10:37; Acts 13:24.
I am at a loss to understand why the MEV, almost alone (I could find no others) capitalizes "Him" in John 1:7.