My atiel has a strange and annoying habit its extremely abnormal! Help?

My atiel Henry (not the one in this pic) has a really weird habbit its hard to explain but here it goes, he would stick the back toe of either foot into his mouth (one at a time) and underbite the sharp part of his beak onto the lower part of his beak and breath in (if u know what i mean) and will continuously do it! He gets silvia all over his mouth and buy doing that he dirties his toes and i have to clean them all the time. He's been doing it scince he was one and he is now four im still trying to find someone to help me, I've talked a bit with my dad and he thinks its some ADD problem because he's quite nervous very anxious and stresses heaps but he looooves people XD and its the worse before he goes to bed its weird and un bird like. I've been trying to figure out the cause for years there is nothing on the internet about it because its sooo different im also very frusterated about that too. Im really hoping one of u lovely people can help me out with this so i can put an end to his obsessive behaviour, thank you heaps im looking forward to some answers :)