Whatever you do don't lie to him. Tell him you feel that it will be better to be friends. If he asks why just tell him the truth, say you don't feel like you two are a good couple so you should go back to being just friends. He won't be happy about all of this, but he'll appreciate your honesty in the end and will most likely be able to be just friends with you.
I've been in your shoes b4. Sometimes its better when you're friends than when you're actually together. Its going to be hard but just try your best to talk to him about it and everything should be fine.;-)
You just tell him the truth. Don't expect him to be thrilled at first but if you are honest and nice it will work out for the better.