Thanks all for your help. The shock factor of gently tapping on the head actually works great as two of you had said it just snaps him out of feeding mode and doesn't seem to cause an major stress.

I did a couple of tests where I pulled him out and handled for about 10 minutes to let him relax. I placed him back in the cage and kept my hands in the cage for a few moments, sometimes taking them out and placing back in near him. There was no response. I closed the lid, walked away for about 2 minutes, came back and opened the lid and he went nuts again So yeah, he definitely has associated the door opening with food time. Just will take a little while to tame I guess! Cheers!