Wow...This is a very hard question...
You just have to honestly tell them. The sooner the better. Before it gets deeper and deeper.
Try to ask him how he feels about you first.
Then ask, do you expect that from me too?
Then ask, what if I don't feel that way?
or any at all? He will know right away you are telling him something. and hope it isn't about breaking up (if he still likes/love you)
tell him honestly about how you feel in this relationship
that you cannot continue this anymore maybe.
or tell him what the problem is and why you want to leave him.
this is hard to explain because I don't know the situation between you two.
Hope you can figure out the rest. I started you off.