Hi there!
I know this sounds simplistic, but if you want to stop, you really do just have to stop taking him through there. By all means stay in the room with him in his cot, and let him cry and figure it out. He won't cry forever, but it may feel like forever. But every time you let him cry for a bit and then give in and take him into the spare room, you're reinforcing the habit. Pick up the teddy and calmly pop it back into his cot (don't hand it to him). Help him to lie down if you feel like he's 'stuck' standing up. When he's finally calm and lying down, take a step towards the door. Then another step. I bet after just a few nights he'll stop bother to wake up, once he realises that the spare room is finished with now.