I think he's fairly happy with the cat flap in each direction now, although being a cat, he'll always prefer it if a door is opened up for him!
When he was first in the house, he'd hide behind the towel rack until I came and found him. After a few days, he progressed to coming out when I used a friendly voice for a while, and then moved on to peering suspiciously at me from the top of the stairs.
He has now got to the stage where he's obviously either hearing the sound of me arriving outside the door, or possibly the key in the front door lock, and getting so close to the front door, that on the last couple of nights I've bopped him on the head, because he's so close to the door when I opened it!
On a couple of other occasions he's been outside, and close enough to the cat flap (which is around the back of the house) that he's heard me coming in, and there's been a "Beep" from the detector and sound of the cat flap rapidly being opened, followed by a dash through the kitchen and the hall, with a friendly meow and dash up to be petted.
I think both instances suggest that he's pretty accustomed to being here, and happy with me now.