If I were to hazard a guess, he fell out of the initial infatuation phase. In general, many people who do OLD don't really know what they want and when things turn into anything more than a short term fling... only do they consider the possibility for long term. Maybe it is harder for you to see this but from a 3rd person, it is clearer in this situation.

OLD in general: validation, attention, and adrenaline. A lot of spontaneity and little consideration for anything that happens past the first month.

Please dont reach out :( you'll end up feeling worse because you're trying to turn around a ship that has already sailed. Just acknowledge that it was a good run! Also, being honest and forthcoming with your feelings is underrated today. At least you're one of those who know what they want and are clear about it. ✌️