How Do I Dump this guy?

This might be kinda long.

Okay so im going out with this guy in a grade lower then me and i like one of my ex's alot more; my ex still likes me. I went out with this guy to give him a chance and now i don't want to go out with him any more. How should i break up with him, I need an excuse about why i cant date him. He thinks everything is going perfect, but in my eyes, nothing is right. Alo because i like my ex more i dont want him to end up more hurt in the long run. It has been a little over 2 weeks and i need to know how i should dump him, i wont do it in person (he would cry) and im not sure if i should tell him the truth about how i like my ex more or give him an excuse about how im failling school and i cant have a bf till after my grades go back up. (i really am failing school, im too distracted with a bf, but i don mind.)