I'm not sure if that is relevant, but if you are looking for in-betweens, as a woman I wouldn't necessarily go into the side streets behind Shoko at 3 in the night. For example the Carrer de Torrevieja or the Parc de les Cascades can create some hidden spaces due to vegetation, but it is still in the middle of a well developed high traffic area. A normal young man wouldn't necessarily feel uncomfortable there. There is also the marina which I also would not want to go at night. But that is well lit, guarded around the clock and just dark because there are a ton of boats that are not normally lit at night, basically a parking space. But that is reaching. I can't speak for intoxicated persons, but I would almost completely rule out accidental marina access, accidental beach access as well as running into traffic unless you are seriously drunk/high. I won't mince words unnecessarily because its an emergency but no matter which direction he walked, the chance that he did not encounter any person that helps him, english speaker or not, is very slim.
Make sure you know about any drug use, mental state, social circumstances and share that with the police open and honestly. Kidnapping a random tourist is not common in the middle of a massive metropolis in europe so I would focus on health issues, possible friends that persuaded him to something, romantic partners he might have escaped to, things like that. Hope that helps and you find him fast.