OK, here are all the possible Red XIII scenarios:
1. He is fully playable, added earlier in the story. This is now highly unlikely because of the current marketing (though it could be a big surprise).
2. He is fully playable, added in at his normal time point - entirely possible. Square Enix will probably not market him as playable due to the false advertising concerns (as he's only available for a short time at the end of the game), we might see a glimpse of him in a trailer
3. He is a guest character for his normal story time period. They're under no obligation to disclose this, and while it would be mildly disappointing, I do feel like most fans would probably understand why they made this choice, given the expectation he'll be a fully fledged party member in part 2.
4. He has been removed from the story entirely - highly unlikely, as it would cause a serious negative backlash and is a massive deviation from the story. But still possible. While they wouldn't "market" this, I feel like something this important is something they would actually announce beforehand to temper expectations.
2 and 3 are the most likely, obviously. To reiterate, they're under no obligation to announce either of these prior to the games release. Having him in the game in either of these capacities and not telling us about it prior to release doesn't hurt or hinder opinion of the game tbh.
So the tl;dr is until they say anything different, assume he's in the game.