And be kind. Heartbreak is hard.

Just be upfront about why

Be honest. That's it.

Letting him hate you a little might work (don't purposely self-sabotage though, obviously). It will be easier for him to move on/accept the break and find some solace. But honesty is the best policy, especially since it's over the phone. Just tell him how you feel and why you want to break up.

I have had to break up with a lot of different people, and it sucks no matter what you do. Believe me. The trying to make them hate you approach is the worst, imo. It helps neither of you, and costs you a possible friend. Be kind, but be aware that you will probably both still feel awful. There really isn't a way around that.

be honest with him...

tell him you tried and you don't want to waste any more of his time

Ugghhh... did he do something to deserve this ? You don't just draw someone in and then tell them ohh I can't do this.

:(