I imagine his previous experiences have led him to believe that standing at the door is enough to get let out when not in his crate. If you've seen him at the door previously and opened it he'll have learnt that thats all you require from him in order to get let out. You need to train him to notify you in another way.
You can buy doggy door bells where he can press a button which will make a noise to notify you or you can train him to bark to be let out. All you need to do to do this is make up his dinner and let him watch you put it outside. Close the door and wait. He'll get fustrated at not being able to get to his dinner so will throw a bit of a strop and will try various behaviours to get the door opened for him. As soon as he make any kind of noise open the door. Over a few days he'll learn that he needs to make a noise to get you to open the door for him.
If he struggles to apply this theory to toileting put his dinner outside but covered or high up so he cant get to it as soon as he runs out. Do the usual routine and When he barks or wines to be let out go outside with him and take him to his toilet area and then wait. Once he been, reward him by giving him his dinner. After a few days he'll be barking, weeing and looking for his dinner.
However Ill warn you that whenever hes hungry you may find hes at the door barking.

For this reason Ive not done this with my dogs. Like Nicola I just get to know their routines and let them out at set points throughout the day.