While I second what they've said, but from a generic perspective, in my experience pretty much all you can do is just be straightforward. Be as tactful as you feel is necessary, but you also don't want to leave room for confusion. Make sure he knows you value his friendship, and that you don't want to lose it, but that you're not looking for anything more from him. There's not necessarily any way to guarantee he'll remain friends with you-- that depends almost entirely on his own decisions. All you can do is not lead him on, even unintentionally. Be honest, be respectful, and hope for the best

smack him with a pan

@bethorien I've found it I'd difficult to maintain friendships if you are abusive to your friends.
@parisqeen and @xvarnah are right of course. Assuming you don't want a relationship with him, you should tell him so. That being said, I think I would handle "he is professing his undying love" and "he seems like he has a crush based on the way I catch him looking at me" a little bit differently.
Ultimately, you probably know the best way to approach the subject better than we do. He is your best friend, right? You probably know the best time and way to bring this up so as to have the conversation go well. Be clear and honest. You may have to accept that he has romantic feelings towards you even though he knows you have no interest in a relationship. These feeling will more than likely fade. Establish reasonable boundaries and hold each other to keeping them. You are both adults, you can handle this I promise.
If you DO have feelings for him, don't lie and say you don't.

but pans fix everything

@bethorien I leave one chat of you talking about frying pans to another chat of you talking about frying pans. I'm starting to suspect you're secretly a salesperson

I'd buy a pan if someone was threatening me with a pan to buy a pan

BUY MY PANS
*scary beth noises*

My entire friend group is based around physical and verbal abuse

@bethorien I think you found your first customer

@bethorien are you PANhandling?

@catfluff quit PANdering

P.s. before this derails, have you gotten the help you wanted?

Yeah, I think you guys are right about being straight forward. That’s the current plan, just being like hey but friendship.
He hasn’t outright said anything like “I have a crush on you” but it’s pretty obvious based on a lot of the other things he says/does. It would take me a while to explain but it’s 100% fact y’all. Even our friends are aware.
I have a feeling direct confession time is coming in the next couple days. Also, yeah, it’s not just because he’s in a relationship, I’m just not looking for a bf right now. So I guess I’ll just have to articulate all of that honestly

Honesty IS the best policy. The thing about honesty is, while sometimes brutal, will eventually bring the most fair outcome. No one is being tricked, lied to, willfully ignored, or led astray. Honesty must come from yourself first (introspect), and then can be transferred to another. You will find that most people will eventually appreciate honesty (retrospect). I hope it goes well, and just know that if you were honest, and the friendship suffers, it’s because he was looking for more to begin with, and never really had the interest to just be friends. He should be honest with himself and see why he is continuing this relationship if he’s seeking another (assuming no polyamorous nature with him exists.).

Hey so base on some outsiders' point of views, it seems like you are somewhat romantically attracted to me. If it's not, that's cool but IF (a huge if here, my dear friend)it's true, I'd have to bluntly tell you to end your feeling. I'm saying this simply bc I want to maintain our friendship. I hope you understand."
It's as soft as it can be tbh. You can just copy and paste that to him if you wanna.
It should work.

@rosalinas That might work, but it might not be the best approach. If my best friend had told me that when I had a crush on her, I would not have taken it well. However, her telling me that she was uninterested in pursuing a romantic relationship with me, but wanted to remain friends worked really well. Telling people to end their feelings is not necessarily the best approach. Being honest a out your feelings 100% a good idea. Expecting people to respect that, super important for healthy friendships. But most people can't just stop feeling a certain way, especially all at once. Telling them they should often results in people being upset as what you have asked isn't really fair. That being said there is a huge difference between having feelings and acting on them, and it is completely reasonable to ask someone not do act on feelings they have towards you as you are not interested in such a relationship

I don't think there's any one way to say it. In my experience it literally depends who you are and who the person you're dealing with is. I've had guys say they're not okay with it, guys say they are okay with it but then proceed to become incredibly petty, and guys who try their hardest to be okay with it. Sometimes it works out, sometimes it doesn't.

I think he’ll handle it pretty well. I’m just praying that he doesn’t take any drastic measures like suddenly breaking up with his gf and then thinking I’ll just agree to go out with him. That would be upsetting for anybody. Good thing that’s the extreme case scenario here.
With that fear considered, anyone got any suggestions for perhaps subtle things I could do before honesty hour arrives?

pan smacking is pretty subtle

No, just come straight out with the truth. Avoid leading him on but don't let that stop you from being yourself around him. I would just try and avoid any unnecessary drama or hints, just be honest.