When is he climbing out of bed? at bedtime or after a solid period of sleep?
If it is right at bedtime you can try placing a mattress next to the bed where you will "go to sleep" with him, to reassure him you're there, it's ok and it's night time. Everytime he tries to climb down, you redirect him and reassure him till eventually he settles and goes to sleep and you can leave the room then. Gradually, as he puts less resistance you can start moving the mattress next to the door till you manage to be out of the room.
If he's waking after a period of sleep and doing so you just redirect him to his bed every single time. The first one you explain shortly it's night time we need to rest in our own beds, I love you, see you tomorrow. For the following, you just redirect him kindly, without words or other interaction. This might take a few exhausting hours but he will eventually get that there's no point in climbing out, since he's not getting anything from you. Even if you show upsseting, he might get that he's getting a reaction out of you and by so, being encouraged to keep on going, so you just calmly direct him again and again being really BORING.
HTH! hang in there!