For years I had not noticed this, since RSV and NRSV perversely translate the prepositional phrase en emoi as "to me," as if this were no different from altering "brothers" to "brothers and sisters."But when it pleased God...to reveal his son in me (apocalypsai ton huion autou en emoi), that I might preach him among the gentiles...
What to make of it? I consulted various commentaries, and noticed that conservative authors sometimes try to explain it away by a fanciful theory of the [en + dative] construction, or by assimilating the usage to the subsequent phrase "among the gentiles," as if that clarified anything.
If you read it plainly, and ignore the other two Pauline reports of his encounter/calling in 1 Cor 9 & 15, and of course the hopelessly fictional account in Acts, you are left with a mystical experience, and not a visionary one. It also does not reference Jesus or Christ specifically, but just God's son, which could be taken a number of ways.
The only recent book I know that examines this passage with insight is Steven Davies' Spirit Possession and the Origins of Christianity. He makes important associations between this mystical event in Paul and the Odes of Solomon, in which the concept of the Messiah is highly mystical and subjective.