Tell him. If he's a good guy, and he really likes you, he'll understand.
There's no magic rule for how to build a relationship, there's no formula. If you're not ready for the emotional intimacy of a relationship, but you and he both feel that you'll be a good match when you ARE ready, then work something out! You may decide to have something casual-but-exclusive, or you may decide to have nothing between you at all... the important thing to remember is that the TWO of you have to DECIDE!
If u go into something you're CLEARLY NOT ready for.... it will just be a DISASTER
why enter in a relationship if youre not ready....stay single then....atleast you have freedom to flirt with other guys that you like too...then decide to be with if you're ready