If I remember correctly, you have to get a little bit of corruption (I think 10 is more than enough) and go to him, talk to him and get close to him, and he will notice the corruption and offer you a cure.
You reject the cure, and then you can have sex with him every time you visit him, slowly corrupting him until you can use the option “Feel his corruption” or something like that. Then you transform him, and they kick you out for corrupting the apprentice of this guy from the bastion.
After a few days away, he will appear in your camp.
Alternatively, you can consume enough Nyx Spawns and talk to Abel, and I think you can corrupt him right away (I haven't played in a long time, so I have no idea if this option is still available).