The 737's engines are very low to the ground. So much so that the housings are flat on the bottom so there is enough ground clearance.
For a taller than average person, they could climb onto the engine, then pull themselves up on top of the engine and onto the wing. Add to the fact no idea what kind of substances he was on so just about anything is likely possible.
The pilots were not quite sure what was going to happen so they shut down the engines when they saw the guy coming towards them.
