keep drinking with him, go on a walk around the town and romance bernerd then visit him (bernerd) at night and you should have an option to both go sleepover in the barn with caleb. Not sure in what order the options appear in but after that you just keep visiting caleb in the morning and the gloryhole will happen sometimes since the scene is all RNG. Think when you go on a walk with caleb he mentions about his 'favorate stall' and that is what triggers the RNG for the gloryhole to happen.