Just luring him with treats might not work. However, you could try to train the dog using the full "power" of positive reinforcement training (aka clicker training). Obtaining behaviours like the one you want can be done with "free shaping" or "luring".
If your not familiar with clicker training, here are a few steps you could try to follow:
- if you have a clicker use one, otherwise just use the word "yes"
- take some treats
- pair the clicker/yes with the treats: without asking anything, just say "yes" then treat, repeat a few times. If the dogs knows a "sit" or another behaviour, ask for it, then yes as he's doing it, then treat
The point here is that the "yes" will be a marker, meaning "what you're doing right now is good, a treat will follow".
- Now move to the flap. Stay close to it and wait
- At first the dog will probably wonder what's going on, do nothing
- First, click and treat as he's looking at the flap
- Then reward (click or "yes" and treat) every steps toward the flap. Follow even the tiniest steps until he's touching the flap, etc.
To do so you can lure it to the flap, but the point is to make him understand that the game is about the flap, not about following a treat.
- Then make sure that he understands that something interesting is on the other side : a toy, whatever
- Continue rewarding actions towards the flap, smelling, touching, going through with one leg, etc. It doesn't matter if he's coming back to you to get the treat, what matters is the moment you click/say "yes".
This might take more than one session, but eventually this is creating the best conditions for him to want to go to the other side by himself. If during the first session the progresses are really slow, then you can "play this game" again later, but on the "good" side of the door, where he's more likely to use the flap. That way he will understand the rules of the game and realises I can win a big reward when he wins.
If he does go in the room by himself make sure he gets a "jackpot" treat.
You can also have a look at this video. The dog there doesn't want to walk on some kind of floor. At first he's really not doing it, but the technique helps him associate the floor and positive feelings.