I feel like I’m getting left behind

Talk to him. You're best friends, you can do that without fear. Tell him you're sorry he got upset but you did not think he would react like that, especially because he started hanging out with other people too. Tell him you don't want you guys to drift apart (actually say it, it's important to state that kind of things) but since you don't share the exact same activities, there is a chance you don't always share the same friends either and that's ok as long as you still have plenty of time together. That is just going to make these moments together even better.
You know, he's probably thinking the same thing as you are, and that's probably why he got upset. If he did not care about the distance settling between you two, he would never have gotten upset.
Now, it's a very normal thing, in any kind of relationship or friendship, to sometimes feel a little less connected. It does not mean it is never gonna be great anymore, it just means you guys are evolving and you need to reajust a little. That's ok and it will break your friendship with him only if you let it. Talk to him, every thing will be better then.

First of all- I'm sorry that you're feeling left out or excluded from your relationship with this friend. Do you have romantic feelings for Snowy? If so it might be a good idea, if you're both single, to discuss your feelings towards them. If you think you're separating from them already, you really don't have much to lose if the feeling isn't mutual. I'm just concerned that if you don't take action soon and make it clear you'd like to keep this person in your life(Romantically or not) You may regret it in the future, missing this person and wondering what you could have done to sustain your time with them. It's something to consider doing, anyways. You're welcome to message me if you'd like to talk more about it- and you can update me on how things progress if you want, as well. I'd love to hear back from you, my friend. Positive vibes are coming your way! Hopefully you gain some clarity soon!