Hey There! Seems like a funny situation... In regards to being able to tell him how you feel and to make sure he knows how you feel you should have a personal one on one with him where you tell him how you feel. Be open and honest ans tell him your view without being too harsh. Maybe don't say he's weird or anything like that but obviously, state that it makes you uncomfortable. I'm unsure whether him daydreaming is related to the problem? But then again he could be into you. If you find him possessive you should talk to him about that too. I personally went through something like this where I was a little too possessive, however, when spoken to by the individual, problems were cleared up and I had the opportunity to change. Maybe you should do the same. Let him know that you believe he is too possessive and tell him to kind of back off in a way. But the most important thing is to give him the chance to change before getting frustrated. Maybe he has mixed signals? Maybe he is under the false impression you have feelings for him? This is okay and completely normal, however, if you talk to him nicely alone (if you are comfortable) you should both be able to express your feelings. The keyword being both. Both people should have the chance to tell eachother how they feel and discuss what to do about it. Hopefully, if he's a decent friend, he should understand that you don't have feelings for him. If he doesn't and continues to make constant moves on you, this should be escalated and other people, parents, guardians etc should be involved. When actually speaking to him, like I said avoid any aggression or frustrated terms. Rather than "I don't want you" perhaps try "I don't feel the same way". Console him and make sure he's okay because, if the guy has feelings for you, it may be difficult to hear that you don't. Tell him that you really love being friends with him! (I assume you do) But make sure that's how you want it to stay. Relationships are a two way thing and they should stay that way
Good luck!