Nala used to always jump on the glass when she wanted to go out, or come in.

I really think sit and stay are VERY important tricks for dogs to know. I tell her to sit through the glass when she's outside wanting to come in. If she doesn't, then she doesn't come in. Nor does she go out if she doesn't sit and stay.

I don't think you need to buy anything. Just some time and patience. Don't let him in or out unless he sit and stays. If he moves when you go to open the door, make him sit again. Do not let him move until you say it's "OK".
Do this EVERYtime you let him in or out.

Nala is much better now. She normally ends up sitting when I go to let her in.