All relationships have unwritten rules and boundaries. Friendships and especially good friendships still have those rules and boundaries that both persons have accepted and followed over the years. These relationship boundaries and arrangements can remain steady for many years.
When you informed your friend that you liked him in a romantic way, you changed the boundaries and agreements in the friendship. When we change the conditions in an existing relationship or friendship, the other party is often perplexed and confused about their next move. It’s their turn to re-evaluate the relationship. It’s like having lunch with the same co-worker for ten years always discussing work, friends, family, current events, etc. then one day announcing you’d like to have a romantic relationship. Many people are uncomfortable when a relationship changes, often electing to reduce the “best friend” to a “friend” status. I suspect your friend has reassessed your relationship and dropped you to a friend level…at least until he decides what to do in the future. You may have lost your “best friend” rating but still retain a friendship.
Now what do you do? Further discussion would likely move him further away, as you suspect. In these situations, it’s helpful to accept your reduced friendship rating and be a friend. As he evaluates his feelings about the friendship, he may return and reestablish the best friend relationship…with the understanding that romance is not involved. In the co-worker example, it’s when the surprised co-worker returns to say “We can continue to go to lunch, but I want no conversations about a romantic or sexual relationship!” At that point, they continue to go to lunch and talk about friends, work, current events, etc. I hope this helps.