If we allow the Bible to interpret itself, it is plain and clear that John DID in fact see this. Therefore, we must conclude that the "he" in the verse, while not excluding Jesus (or perhaps others as well), is specifically focused on John. It seems that John was promised a sign, and God provided this as the sign that indeed this was the Messiah.
And John bare record, saying, I saw the Spirit descending from heaven
like a dove, and it abode upon him.
And I knew him not: but he that sent me to baptize with water, the same
said unto me, Upon whom thou shalt see the Spirit descending, and
remaining on him, the same is he which baptizeth with the Holy Ghost.
And I saw, and bare record that this is the Son of God. (John 1:32-34)
Grammatically, Mark 1:10 is ambiguous. The "he" might refer either to John or to Jesus. Had the writer used "they" in an attempt to include both of them, the third-person plural would actually open the field to potentially include all present. But that John is the special focus of the text is made clear in this parallel passage from the book of John. Since John tells us that John had seen this, applying Mark's "he" to Jesus only would make the interpretation untrue.