How do you tell your best friend you love them?

I he doesnt feel the same you shouldnt tell him you love him. It could ruin the friendship.

I'm sure you don't want to hear this but that sounds like a real red flag to me.

I'm a guy and when I'm around my female friends, if I'm uninterested in that person romantically. I'll be sure not to say anything that might be interpreted as a flirt or advancement. Most reasonable guys will have a conscious filter to know not to say anything that might give the wrong idea unless they're interested.

But this guy knows you are interested in him and he says the feeling isn't mutual but yet he heavily flirts with you, playing with your emotions and basically saying things that typically boyfriends would say.

Either the guy is very oblivious to what he says which is unlikely, or he genuinely is interested and just for whatever reason cannot commit, possibly due to work or a separate commitment or whatever it may be or he could be playing with your feelings because he enjoys you crushing over him without him having to have any commitment.

Ultimately, I would say to confront him next time he says something really flirtatious and asks if he genuinely feels that way and then go from there.

is he real?
or
is he just imaginary?

The ability to communicate to someone that you actually love and care for them is a common issue. He may be telling you that in his way - and you may be telling him that in your way... and you both are missing it because your "love languages" are different. Kinda like someone who speaks only French talking to someone who speaks only Russian.

I suggest you get and read "The 5 Love Languages" by Gary Chapman (there is a teenage, and senior versions as well).

It works best if you get 2 copies and ask them to read it too (and one of you likely reads faster than the other...; but all interested should be able to get through the book in 2 weeks or less).

When I read it I learned so much - and realized exactly why most of my previous relationships failed. My GF at the time read it and found the same - and told me: "so there is a real chance this would work out between us" (we then married - coming up on 9 years in April).

Now if he is not willing to read the book to improve his relationship with you... and other future GF's if the two of you don't match in other ways. Run, don't walk away; and never look back. Find someone who is willing to work on making it work.

He could feel that he isn't good enough due to insecurity or that the two of you aren't compatible romantically even if there's feelings there. Sometimes a friendship with someone can work great but when you try to be together romantically all you do is argue or you dont understand each others love languages or something else.
So the risk is you ruin the friendship but if it's a risk you're willing to take then do it.

Keeping options open and you on the hook.

You don't.