When im talking about speed of light i am assuming it will be in a perfect vacuum. If this was in ambient under normal conditions, a train going the speed of light would ionize all the air around it causing insane levels of heat.
So with the thought of it moving in a vacuum, if you look at the portal on a frame by frame basis every nano second you would see either
1 nano second in his entire body is within some imaginary dimension between the 2 spaces
The body gets infinitely squeezed in 1 space turning them into a mini black hole
They leave the portal at the same rate they came in
These are my 3 options, i dont see how it can be any other way.