Depends how far you are willing to go to get his number back. Each method below does get increasingly stalker'ish. So it's really up to you.

A phone professional could probably recover the text, unless your phone memory is maxed out and has been overwritten. A quick Google search on how to recover deleted texts on Android revealed a method involving a recycle bin built into the android text application. When deleting files, they are seldom permanently deleted, they are instead marked to be overwritten if your device needs the storage space.

If you have any pictures of his face, you can do a reverse Google image search and possibly find his Facebook or linked in profile or something... Or simply search for him on Facebook

If you know his name and town he lives in, or better yet, his favourite activities. You could phone a couple establishments near him and leave a message.

If you know where he works then could try that too. But don't straight up tell the receptionist your story, just ask if you can speak to Mr X. This is technically social engineering so choose your words wisely.

I had some school friends that would love to pull off something like this. Probably still would if the opportunity came up.

In fact I suspect my cousin's pulled a similar trick on me when I was young. There was a burned out abandoned house walking distance from their house. And we found some tapes in a hole in the ground near a tree. Kinda seemed inspired by Blair witch project. The whole exploration seems like they were trying to hype things up a bit... One day when I see them again for a beer, I will ask. Haha