I agree - at a minimum dress pants and polo, dress shirt and a tie if you can manage it. You want to be someone he can visualize as a cadet at the academy. While the MALO doesn't have as much input for WP as the liaison's do for other academies, I think there are two main points:
1. The MALO is there to help you and answer your questions and make sure you know what you're getting into. Unless you really, really screw up (swear a lot, tell him you're only going for the free education), he is your advocate. But remember that he can blackball you (effectively), so just treat him with respect. He will realize this may be your first interview and won't rate you negatively because you didn't call him sir, or were nervous or slouched or anything like that - just basically don't do anything stupid!
2. The MALO CAN be a strong advocate for you. He can call WP and check on your files, give you an idea of where you stand compared to others, give you a heads up of where you need to improve and of course get answers to your questions. If you do make a strong positive impression, he's far more likely to try to go out of his way to help you out.
So it's certainly not a "make or break" interview, but can definitely be of benefit to you in the long run!